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2812 lines
103 KiB
2812 lines
103 KiB
.. _cgroup-v2: |
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================ |
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Control Group v2 |
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================ |
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:Date: October, 2015 |
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:Author: Tejun Heo <[email protected]> |
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This is the authoritative documentation on the design, interface and |
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conventions of cgroup v2. It describes all userland-visible aspects |
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of cgroup including core and specific controller behaviors. All |
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future changes must be reflected in this document. Documentation for |
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v1 is available under :ref:`Documentation/admin-guide/cgroup-v1/index.rst <cgroup-v1>`. |
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.. CONTENTS |
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1. Introduction |
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1-1. Terminology |
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1-2. What is cgroup? |
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2. Basic Operations |
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2-1. Mounting |
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2-2. Organizing Processes and Threads |
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2-2-1. Processes |
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2-2-2. Threads |
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2-3. [Un]populated Notification |
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2-4. Controlling Controllers |
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2-4-1. Enabling and Disabling |
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2-4-2. Top-down Constraint |
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2-4-3. No Internal Process Constraint |
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2-5. Delegation |
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2-5-1. Model of Delegation |
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2-5-2. Delegation Containment |
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2-6. Guidelines |
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2-6-1. Organize Once and Control |
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2-6-2. Avoid Name Collisions |
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3. Resource Distribution Models |
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3-1. Weights |
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3-2. Limits |
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3-3. Protections |
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3-4. Allocations |
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4. Interface Files |
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4-1. Format |
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4-2. Conventions |
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4-3. Core Interface Files |
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5. Controllers |
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5-1. CPU |
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5-1-1. CPU Interface Files |
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5-2. Memory |
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5-2-1. Memory Interface Files |
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5-2-2. Usage Guidelines |
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5-2-3. Memory Ownership |
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5-3. IO |
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5-3-1. IO Interface Files |
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5-3-2. Writeback |
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5-3-3. IO Latency |
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5-3-3-1. How IO Latency Throttling Works |
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5-3-3-2. IO Latency Interface Files |
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5-3-4. IO Priority |
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5-4. PID |
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5-4-1. PID Interface Files |
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5-5. Cpuset |
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5.5-1. Cpuset Interface Files |
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5-6. Device |
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5-7. RDMA |
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5-7-1. RDMA Interface Files |
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5-8. HugeTLB |
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5.8-1. HugeTLB Interface Files |
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5-9. Misc |
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5.9-1 Miscellaneous cgroup Interface Files |
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5.9-2 Migration and Ownership |
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5-10. Others |
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5-10-1. perf_event |
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5-N. Non-normative information |
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5-N-1. CPU controller root cgroup process behaviour |
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5-N-2. IO controller root cgroup process behaviour |
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6. Namespace |
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6-1. Basics |
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6-2. The Root and Views |
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6-3. Migration and setns(2) |
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6-4. Interaction with Other Namespaces |
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P. Information on Kernel Programming |
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P-1. Filesystem Support for Writeback |
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D. Deprecated v1 Core Features |
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R. Issues with v1 and Rationales for v2 |
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R-1. Multiple Hierarchies |
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R-2. Thread Granularity |
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R-3. Competition Between Inner Nodes and Threads |
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R-4. Other Interface Issues |
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R-5. Controller Issues and Remedies |
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R-5-1. Memory |
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Introduction |
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============ |
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Terminology |
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----------- |
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"cgroup" stands for "control group" and is never capitalized. The |
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singular form is used to designate the whole feature and also as a |
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qualifier as in "cgroup controllers". When explicitly referring to |
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multiple individual control groups, the plural form "cgroups" is used. |
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What is cgroup? |
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--------------- |
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cgroup is a mechanism to organize processes hierarchically and |
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distribute system resources along the hierarchy in a controlled and |
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configurable manner. |
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cgroup is largely composed of two parts - the core and controllers. |
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cgroup core is primarily responsible for hierarchically organizing |
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processes. A cgroup controller is usually responsible for |
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distributing a specific type of system resource along the hierarchy |
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although there are utility controllers which serve purposes other than |
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resource distribution. |
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cgroups form a tree structure and every process in the system belongs |
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to one and only one cgroup. All threads of a process belong to the |
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same cgroup. On creation, all processes are put in the cgroup that |
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the parent process belongs to at the time. A process can be migrated |
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to another cgroup. Migration of a process doesn't affect already |
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existing descendant processes. |
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Following certain structural constraints, controllers may be enabled or |
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disabled selectively on a cgroup. All controller behaviors are |
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hierarchical - if a controller is enabled on a cgroup, it affects all |
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processes which belong to the cgroups consisting the inclusive |
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sub-hierarchy of the cgroup. When a controller is enabled on a nested |
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cgroup, it always restricts the resource distribution further. The |
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restrictions set closer to the root in the hierarchy can not be |
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overridden from further away. |
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Basic Operations |
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================ |
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Mounting |
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-------- |
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Unlike v1, cgroup v2 has only single hierarchy. The cgroup v2 |
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hierarchy can be mounted with the following mount command:: |
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# mount -t cgroup2 none $MOUNT_POINT |
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cgroup2 filesystem has the magic number 0x63677270 ("cgrp"). All |
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controllers which support v2 and are not bound to a v1 hierarchy are |
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automatically bound to the v2 hierarchy and show up at the root. |
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Controllers which are not in active use in the v2 hierarchy can be |
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bound to other hierarchies. This allows mixing v2 hierarchy with the |
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legacy v1 multiple hierarchies in a fully backward compatible way. |
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A controller can be moved across hierarchies only after the controller |
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is no longer referenced in its current hierarchy. Because per-cgroup |
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controller states are destroyed asynchronously and controllers may |
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have lingering references, a controller may not show up immediately on |
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the v2 hierarchy after the final umount of the previous hierarchy. |
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Similarly, a controller should be fully disabled to be moved out of |
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the unified hierarchy and it may take some time for the disabled |
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controller to become available for other hierarchies; furthermore, due |
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to inter-controller dependencies, other controllers may need to be |
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disabled too. |
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While useful for development and manual configurations, moving |
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controllers dynamically between the v2 and other hierarchies is |
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strongly discouraged for production use. It is recommended to decide |
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the hierarchies and controller associations before starting using the |
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controllers after system boot. |
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During transition to v2, system management software might still |
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automount the v1 cgroup filesystem and so hijack all controllers |
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during boot, before manual intervention is possible. To make testing |
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and experimenting easier, the kernel parameter cgroup_no_v1= allows |
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disabling controllers in v1 and make them always available in v2. |
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cgroup v2 currently supports the following mount options. |
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nsdelegate |
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Consider cgroup namespaces as delegation boundaries. This |
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option is system wide and can only be set on mount or modified |
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through remount from the init namespace. The mount option is |
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ignored on non-init namespace mounts. Please refer to the |
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Delegation section for details. |
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memory_localevents |
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Only populate memory.events with data for the current cgroup, |
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and not any subtrees. This is legacy behaviour, the default |
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behaviour without this option is to include subtree counts. |
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This option is system wide and can only be set on mount or |
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modified through remount from the init namespace. The mount |
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option is ignored on non-init namespace mounts. |
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memory_recursiveprot |
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Recursively apply memory.min and memory.low protection to |
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entire subtrees, without requiring explicit downward |
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propagation into leaf cgroups. This allows protecting entire |
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subtrees from one another, while retaining free competition |
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within those subtrees. This should have been the default |
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behavior but is a mount-option to avoid regressing setups |
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relying on the original semantics (e.g. specifying bogusly |
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high 'bypass' protection values at higher tree levels). |
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Organizing Processes and Threads |
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-------------------------------- |
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Processes |
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~~~~~~~~~ |
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Initially, only the root cgroup exists to which all processes belong. |
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A child cgroup can be created by creating a sub-directory:: |
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# mkdir $CGROUP_NAME |
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A given cgroup may have multiple child cgroups forming a tree |
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structure. Each cgroup has a read-writable interface file |
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"cgroup.procs". When read, it lists the PIDs of all processes which |
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belong to the cgroup one-per-line. The PIDs are not ordered and the |
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same PID may show up more than once if the process got moved to |
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another cgroup and then back or the PID got recycled while reading. |
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A process can be migrated into a cgroup by writing its PID to the |
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target cgroup's "cgroup.procs" file. Only one process can be migrated |
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on a single write(2) call. If a process is composed of multiple |
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threads, writing the PID of any thread migrates all threads of the |
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process. |
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When a process forks a child process, the new process is born into the |
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cgroup that the forking process belongs to at the time of the |
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operation. After exit, a process stays associated with the cgroup |
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that it belonged to at the time of exit until it's reaped; however, a |
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zombie process does not appear in "cgroup.procs" and thus can't be |
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moved to another cgroup. |
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A cgroup which doesn't have any children or live processes can be |
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destroyed by removing the directory. Note that a cgroup which doesn't |
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have any children and is associated only with zombie processes is |
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considered empty and can be removed:: |
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# rmdir $CGROUP_NAME |
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"/proc/$PID/cgroup" lists a process's cgroup membership. If legacy |
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cgroup is in use in the system, this file may contain multiple lines, |
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one for each hierarchy. The entry for cgroup v2 is always in the |
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format "0::$PATH":: |
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# cat /proc/842/cgroup |
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... |
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0::/test-cgroup/test-cgroup-nested |
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If the process becomes a zombie and the cgroup it was associated with |
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is removed subsequently, " (deleted)" is appended to the path:: |
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# cat /proc/842/cgroup |
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... |
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0::/test-cgroup/test-cgroup-nested (deleted) |
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Threads |
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~~~~~~~ |
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cgroup v2 supports thread granularity for a subset of controllers to |
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support use cases requiring hierarchical resource distribution across |
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the threads of a group of processes. By default, all threads of a |
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process belong to the same cgroup, which also serves as the resource |
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domain to host resource consumptions which are not specific to a |
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process or thread. The thread mode allows threads to be spread across |
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a subtree while still maintaining the common resource domain for them. |
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Controllers which support thread mode are called threaded controllers. |
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The ones which don't are called domain controllers. |
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Marking a cgroup threaded makes it join the resource domain of its |
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parent as a threaded cgroup. The parent may be another threaded |
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cgroup whose resource domain is further up in the hierarchy. The root |
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of a threaded subtree, that is, the nearest ancestor which is not |
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threaded, is called threaded domain or thread root interchangeably and |
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serves as the resource domain for the entire subtree. |
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Inside a threaded subtree, threads of a process can be put in |
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different cgroups and are not subject to the no internal process |
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constraint - threaded controllers can be enabled on non-leaf cgroups |
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whether they have threads in them or not. |
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As the threaded domain cgroup hosts all the domain resource |
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consumptions of the subtree, it is considered to have internal |
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resource consumptions whether there are processes in it or not and |
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can't have populated child cgroups which aren't threaded. Because the |
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root cgroup is not subject to no internal process constraint, it can |
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serve both as a threaded domain and a parent to domain cgroups. |
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The current operation mode or type of the cgroup is shown in the |
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"cgroup.type" file which indicates whether the cgroup is a normal |
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domain, a domain which is serving as the domain of a threaded subtree, |
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or a threaded cgroup. |
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On creation, a cgroup is always a domain cgroup and can be made |
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threaded by writing "threaded" to the "cgroup.type" file. The |
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operation is single direction:: |
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# echo threaded > cgroup.type |
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Once threaded, the cgroup can't be made a domain again. To enable the |
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thread mode, the following conditions must be met. |
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- As the cgroup will join the parent's resource domain. The parent |
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must either be a valid (threaded) domain or a threaded cgroup. |
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- When the parent is an unthreaded domain, it must not have any domain |
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controllers enabled or populated domain children. The root is |
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exempt from this requirement. |
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Topology-wise, a cgroup can be in an invalid state. Please consider |
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the following topology:: |
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A (threaded domain) - B (threaded) - C (domain, just created) |
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C is created as a domain but isn't connected to a parent which can |
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host child domains. C can't be used until it is turned into a |
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threaded cgroup. "cgroup.type" file will report "domain (invalid)" in |
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these cases. Operations which fail due to invalid topology use |
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EOPNOTSUPP as the errno. |
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A domain cgroup is turned into a threaded domain when one of its child |
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cgroup becomes threaded or threaded controllers are enabled in the |
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"cgroup.subtree_control" file while there are processes in the cgroup. |
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A threaded domain reverts to a normal domain when the conditions |
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clear. |
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When read, "cgroup.threads" contains the list of the thread IDs of all |
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threads in the cgroup. Except that the operations are per-thread |
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instead of per-process, "cgroup.threads" has the same format and |
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behaves the same way as "cgroup.procs". While "cgroup.threads" can be |
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written to in any cgroup, as it can only move threads inside the same |
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threaded domain, its operations are confined inside each threaded |
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subtree. |
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The threaded domain cgroup serves as the resource domain for the whole |
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subtree, and, while the threads can be scattered across the subtree, |
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all the processes are considered to be in the threaded domain cgroup. |
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"cgroup.procs" in a threaded domain cgroup contains the PIDs of all |
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processes in the subtree and is not readable in the subtree proper. |
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However, "cgroup.procs" can be written to from anywhere in the subtree |
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to migrate all threads of the matching process to the cgroup. |
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Only threaded controllers can be enabled in a threaded subtree. When |
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a threaded controller is enabled inside a threaded subtree, it only |
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accounts for and controls resource consumptions associated with the |
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threads in the cgroup and its descendants. All consumptions which |
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aren't tied to a specific thread belong to the threaded domain cgroup. |
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Because a threaded subtree is exempt from no internal process |
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constraint, a threaded controller must be able to handle competition |
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between threads in a non-leaf cgroup and its child cgroups. Each |
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threaded controller defines how such competitions are handled. |
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[Un]populated Notification |
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-------------------------- |
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Each non-root cgroup has a "cgroup.events" file which contains |
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"populated" field indicating whether the cgroup's sub-hierarchy has |
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live processes in it. Its value is 0 if there is no live process in |
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the cgroup and its descendants; otherwise, 1. poll and [id]notify |
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events are triggered when the value changes. This can be used, for |
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example, to start a clean-up operation after all processes of a given |
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sub-hierarchy have exited. The populated state updates and |
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notifications are recursive. Consider the following sub-hierarchy |
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where the numbers in the parentheses represent the numbers of processes |
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in each cgroup:: |
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A(4) - B(0) - C(1) |
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\ D(0) |
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A, B and C's "populated" fields would be 1 while D's 0. After the one |
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process in C exits, B and C's "populated" fields would flip to "0" and |
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file modified events will be generated on the "cgroup.events" files of |
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both cgroups. |
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Controlling Controllers |
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----------------------- |
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Enabling and Disabling |
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~~~~~~~~~~~~~~~~~~~~~~ |
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Each cgroup has a "cgroup.controllers" file which lists all |
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controllers available for the cgroup to enable:: |
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# cat cgroup.controllers |
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cpu io memory |
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No controller is enabled by default. Controllers can be enabled and |
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disabled by writing to the "cgroup.subtree_control" file:: |
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# echo "+cpu +memory -io" > cgroup.subtree_control |
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Only controllers which are listed in "cgroup.controllers" can be |
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enabled. When multiple operations are specified as above, either they |
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all succeed or fail. If multiple operations on the same controller |
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are specified, the last one is effective. |
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Enabling a controller in a cgroup indicates that the distribution of |
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the target resource across its immediate children will be controlled. |
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Consider the following sub-hierarchy. The enabled controllers are |
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listed in parentheses:: |
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A(cpu,memory) - B(memory) - C() |
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\ D() |
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As A has "cpu" and "memory" enabled, A will control the distribution |
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of CPU cycles and memory to its children, in this case, B. As B has |
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"memory" enabled but not "CPU", C and D will compete freely on CPU |
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cycles but their division of memory available to B will be controlled. |
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As a controller regulates the distribution of the target resource to |
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the cgroup's children, enabling it creates the controller's interface |
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files in the child cgroups. In the above example, enabling "cpu" on B |
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would create the "cpu." prefixed controller interface files in C and |
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D. Likewise, disabling "memory" from B would remove the "memory." |
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prefixed controller interface files from C and D. This means that the |
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controller interface files - anything which doesn't start with |
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"cgroup." are owned by the parent rather than the cgroup itself. |
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Top-down Constraint |
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~~~~~~~~~~~~~~~~~~~ |
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Resources are distributed top-down and a cgroup can further distribute |
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a resource only if the resource has been distributed to it from the |
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parent. This means that all non-root "cgroup.subtree_control" files |
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can only contain controllers which are enabled in the parent's |
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"cgroup.subtree_control" file. A controller can be enabled only if |
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the parent has the controller enabled and a controller can't be |
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disabled if one or more children have it enabled. |
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No Internal Process Constraint |
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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Non-root cgroups can distribute domain resources to their children |
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only when they don't have any processes of their own. In other words, |
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only domain cgroups which don't contain any processes can have domain |
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controllers enabled in their "cgroup.subtree_control" files. |
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This guarantees that, when a domain controller is looking at the part |
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of the hierarchy which has it enabled, processes are always only on |
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the leaves. This rules out situations where child cgroups compete |
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against internal processes of the parent. |
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The root cgroup is exempt from this restriction. Root contains |
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processes and anonymous resource consumption which can't be associated |
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with any other cgroups and requires special treatment from most |
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controllers. How resource consumption in the root cgroup is governed |
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is up to each controller (for more information on this topic please |
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refer to the Non-normative information section in the Controllers |
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chapter). |
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Note that the restriction doesn't get in the way if there is no |
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enabled controller in the cgroup's "cgroup.subtree_control". This is |
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important as otherwise it wouldn't be possible to create children of a |
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populated cgroup. To control resource distribution of a cgroup, the |
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cgroup must create children and transfer all its processes to the |
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children before enabling controllers in its "cgroup.subtree_control" |
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file. |
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Delegation |
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---------- |
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Model of Delegation |
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~~~~~~~~~~~~~~~~~~~ |
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A cgroup can be delegated in two ways. First, to a less privileged |
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user by granting write access of the directory and its "cgroup.procs", |
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"cgroup.threads" and "cgroup.subtree_control" files to the user. |
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Second, if the "nsdelegate" mount option is set, automatically to a |
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cgroup namespace on namespace creation. |
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Because the resource control interface files in a given directory |
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control the distribution of the parent's resources, the delegatee |
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shouldn't be allowed to write to them. For the first method, this is |
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achieved by not granting access to these files. For the second, the |
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kernel rejects writes to all files other than "cgroup.procs" and |
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"cgroup.subtree_control" on a namespace root from inside the |
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namespace. |
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The end results are equivalent for both delegation types. Once |
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delegated, the user can build sub-hierarchy under the directory, |
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organize processes inside it as it sees fit and further distribute the |
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resources it received from the parent. The limits and other settings |
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of all resource controllers are hierarchical and regardless of what |
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happens in the delegated sub-hierarchy, nothing can escape the |
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resource restrictions imposed by the parent. |
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Currently, cgroup doesn't impose any restrictions on the number of |
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cgroups in or nesting depth of a delegated sub-hierarchy; however, |
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this may be limited explicitly in the future. |
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Delegation Containment |
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~~~~~~~~~~~~~~~~~~~~~~ |
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A delegated sub-hierarchy is contained in the sense that processes |
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can't be moved into or out of the sub-hierarchy by the delegatee. |
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For delegations to a less privileged user, this is achieved by |
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requiring the following conditions for a process with a non-root euid |
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to migrate a target process into a cgroup by writing its PID to the |
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"cgroup.procs" file. |
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- The writer must have write access to the "cgroup.procs" file. |
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- The writer must have write access to the "cgroup.procs" file of the |
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common ancestor of the source and destination cgroups. |
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The above two constraints ensure that while a delegatee may migrate |
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processes around freely in the delegated sub-hierarchy it can't pull |
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in from or push out to outside the sub-hierarchy. |
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For an example, let's assume cgroups C0 and C1 have been delegated to |
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user U0 who created C00, C01 under C0 and C10 under C1 as follows and |
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all processes under C0 and C1 belong to U0:: |
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~~~~~~~~~~~~~ - C0 - C00 |
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~ cgroup ~ \ C01 |
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~ hierarchy ~ |
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~~~~~~~~~~~~~ - C1 - C10 |
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Let's also say U0 wants to write the PID of a process which is |
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currently in C10 into "C00/cgroup.procs". U0 has write access to the |
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file; however, the common ancestor of the source cgroup C10 and the |
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destination cgroup C00 is above the points of delegation and U0 would |
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not have write access to its "cgroup.procs" files and thus the write |
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will be denied with -EACCES. |
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For delegations to namespaces, containment is achieved by requiring |
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that both the source and destination cgroups are reachable from the |
|
namespace of the process which is attempting the migration. If either |
|
is not reachable, the migration is rejected with -ENOENT. |
|
|
|
|
|
Guidelines |
|
---------- |
|
|
|
Organize Once and Control |
|
~~~~~~~~~~~~~~~~~~~~~~~~~ |
|
|
|
Migrating a process across cgroups is a relatively expensive operation |
|
and stateful resources such as memory are not moved together with the |
|
process. This is an explicit design decision as there often exist |
|
inherent trade-offs between migration and various hot paths in terms |
|
of synchronization cost. |
|
|
|
As such, migrating processes across cgroups frequently as a means to |
|
apply different resource restrictions is discouraged. A workload |
|
should be assigned to a cgroup according to the system's logical and |
|
resource structure once on start-up. Dynamic adjustments to resource |
|
distribution can be made by changing controller configuration through |
|
the interface files. |
|
|
|
|
|
Avoid Name Collisions |
|
~~~~~~~~~~~~~~~~~~~~~ |
|
|
|
Interface files for a cgroup and its children cgroups occupy the same |
|
directory and it is possible to create children cgroups which collide |
|
with interface files. |
|
|
|
All cgroup core interface files are prefixed with "cgroup." and each |
|
controller's interface files are prefixed with the controller name and |
|
a dot. A controller's name is composed of lower case alphabets and |
|
'_'s but never begins with an '_' so it can be used as the prefix |
|
character for collision avoidance. Also, interface file names won't |
|
start or end with terms which are often used in categorizing workloads |
|
such as job, service, slice, unit or workload. |
|
|
|
cgroup doesn't do anything to prevent name collisions and it's the |
|
user's responsibility to avoid them. |
|
|
|
|
|
Resource Distribution Models |
|
============================ |
|
|
|
cgroup controllers implement several resource distribution schemes |
|
depending on the resource type and expected use cases. This section |
|
describes major schemes in use along with their expected behaviors. |
|
|
|
|
|
Weights |
|
------- |
|
|
|
A parent's resource is distributed by adding up the weights of all |
|
active children and giving each the fraction matching the ratio of its |
|
weight against the sum. As only children which can make use of the |
|
resource at the moment participate in the distribution, this is |
|
work-conserving. Due to the dynamic nature, this model is usually |
|
used for stateless resources. |
|
|
|
All weights are in the range [1, 10000] with the default at 100. This |
|
allows symmetric multiplicative biases in both directions at fine |
|
enough granularity while staying in the intuitive range. |
|
|
|
As long as the weight is in range, all configuration combinations are |
|
valid and there is no reason to reject configuration changes or |
|
process migrations. |
|
|
|
"cpu.weight" proportionally distributes CPU cycles to active children |
|
and is an example of this type. |
|
|
|
|
|
Limits |
|
------ |
|
|
|
A child can only consume upto the configured amount of the resource. |
|
Limits can be over-committed - the sum of the limits of children can |
|
exceed the amount of resource available to the parent. |
|
|
|
Limits are in the range [0, max] and defaults to "max", which is noop. |
|
|
|
As limits can be over-committed, all configuration combinations are |
|
valid and there is no reason to reject configuration changes or |
|
process migrations. |
|
|
|
"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume |
|
on an IO device and is an example of this type. |
|
|
|
|
|
Protections |
|
----------- |
|
|
|
A cgroup is protected upto the configured amount of the resource |
|
as long as the usages of all its ancestors are under their |
|
protected levels. Protections can be hard guarantees or best effort |
|
soft boundaries. Protections can also be over-committed in which case |
|
only upto the amount available to the parent is protected among |
|
children. |
|
|
|
Protections are in the range [0, max] and defaults to 0, which is |
|
noop. |
|
|
|
As protections can be over-committed, all configuration combinations |
|
are valid and there is no reason to reject configuration changes or |
|
process migrations. |
|
|
|
"memory.low" implements best-effort memory protection and is an |
|
example of this type. |
|
|
|
|
|
Allocations |
|
----------- |
|
|
|
A cgroup is exclusively allocated a certain amount of a finite |
|
resource. Allocations can't be over-committed - the sum of the |
|
allocations of children can not exceed the amount of resource |
|
available to the parent. |
|
|
|
Allocations are in the range [0, max] and defaults to 0, which is no |
|
resource. |
|
|
|
As allocations can't be over-committed, some configuration |
|
combinations are invalid and should be rejected. Also, if the |
|
resource is mandatory for execution of processes, process migrations |
|
may be rejected. |
|
|
|
"cpu.rt.max" hard-allocates realtime slices and is an example of this |
|
type. |
|
|
|
|
|
Interface Files |
|
=============== |
|
|
|
Format |
|
------ |
|
|
|
All interface files should be in one of the following formats whenever |
|
possible:: |
|
|
|
New-line separated values |
|
(when only one value can be written at once) |
|
|
|
VAL0\n |
|
VAL1\n |
|
... |
|
|
|
Space separated values |
|
(when read-only or multiple values can be written at once) |
|
|
|
VAL0 VAL1 ...\n |
|
|
|
Flat keyed |
|
|
|
KEY0 VAL0\n |
|
KEY1 VAL1\n |
|
... |
|
|
|
Nested keyed |
|
|
|
KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01... |
|
KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11... |
|
... |
|
|
|
For a writable file, the format for writing should generally match |
|
reading; however, controllers may allow omitting later fields or |
|
implement restricted shortcuts for most common use cases. |
|
|
|
For both flat and nested keyed files, only the values for a single key |
|
can be written at a time. For nested keyed files, the sub key pairs |
|
may be specified in any order and not all pairs have to be specified. |
|
|
|
|
|
Conventions |
|
----------- |
|
|
|
- Settings for a single feature should be contained in a single file. |
|
|
|
- The root cgroup should be exempt from resource control and thus |
|
shouldn't have resource control interface files. |
|
|
|
- The default time unit is microseconds. If a different unit is ever |
|
used, an explicit unit suffix must be present. |
|
|
|
- A parts-per quantity should use a percentage decimal with at least |
|
two digit fractional part - e.g. 13.40. |
|
|
|
- If a controller implements weight based resource distribution, its |
|
interface file should be named "weight" and have the range [1, |
|
10000] with 100 as the default. The values are chosen to allow |
|
enough and symmetric bias in both directions while keeping it |
|
intuitive (the default is 100%). |
|
|
|
- If a controller implements an absolute resource guarantee and/or |
|
limit, the interface files should be named "min" and "max" |
|
respectively. If a controller implements best effort resource |
|
guarantee and/or limit, the interface files should be named "low" |
|
and "high" respectively. |
|
|
|
In the above four control files, the special token "max" should be |
|
used to represent upward infinity for both reading and writing. |
|
|
|
- If a setting has a configurable default value and keyed specific |
|
overrides, the default entry should be keyed with "default" and |
|
appear as the first entry in the file. |
|
|
|
The default value can be updated by writing either "default $VAL" or |
|
"$VAL". |
|
|
|
When writing to update a specific override, "default" can be used as |
|
the value to indicate removal of the override. Override entries |
|
with "default" as the value must not appear when read. |
|
|
|
For example, a setting which is keyed by major:minor device numbers |
|
with integer values may look like the following:: |
|
|
|
# cat cgroup-example-interface-file |
|
default 150 |
|
8:0 300 |
|
|
|
The default value can be updated by:: |
|
|
|
# echo 125 > cgroup-example-interface-file |
|
|
|
or:: |
|
|
|
# echo "default 125" > cgroup-example-interface-file |
|
|
|
An override can be set by:: |
|
|
|
# echo "8:16 170" > cgroup-example-interface-file |
|
|
|
and cleared by:: |
|
|
|
# echo "8:0 default" > cgroup-example-interface-file |
|
# cat cgroup-example-interface-file |
|
default 125 |
|
8:16 170 |
|
|
|
- For events which are not very high frequency, an interface file |
|
"events" should be created which lists event key value pairs. |
|
Whenever a notifiable event happens, file modified event should be |
|
generated on the file. |
|
|
|
|
|
Core Interface Files |
|
-------------------- |
|
|
|
All cgroup core files are prefixed with "cgroup." |
|
|
|
cgroup.type |
|
A read-write single value file which exists on non-root |
|
cgroups. |
|
|
|
When read, it indicates the current type of the cgroup, which |
|
can be one of the following values. |
|
|
|
- "domain" : A normal valid domain cgroup. |
|
|
|
- "domain threaded" : A threaded domain cgroup which is |
|
serving as the root of a threaded subtree. |
|
|
|
- "domain invalid" : A cgroup which is in an invalid state. |
|
It can't be populated or have controllers enabled. It may |
|
be allowed to become a threaded cgroup. |
|
|
|
- "threaded" : A threaded cgroup which is a member of a |
|
threaded subtree. |
|
|
|
A cgroup can be turned into a threaded cgroup by writing |
|
"threaded" to this file. |
|
|
|
cgroup.procs |
|
A read-write new-line separated values file which exists on |
|
all cgroups. |
|
|
|
When read, it lists the PIDs of all processes which belong to |
|
the cgroup one-per-line. The PIDs are not ordered and the |
|
same PID may show up more than once if the process got moved |
|
to another cgroup and then back or the PID got recycled while |
|
reading. |
|
|
|
A PID can be written to migrate the process associated with |
|
the PID to the cgroup. The writer should match all of the |
|
following conditions. |
|
|
|
- It must have write access to the "cgroup.procs" file. |
|
|
|
- It must have write access to the "cgroup.procs" file of the |
|
common ancestor of the source and destination cgroups. |
|
|
|
When delegating a sub-hierarchy, write access to this file |
|
should be granted along with the containing directory. |
|
|
|
In a threaded cgroup, reading this file fails with EOPNOTSUPP |
|
as all the processes belong to the thread root. Writing is |
|
supported and moves every thread of the process to the cgroup. |
|
|
|
cgroup.threads |
|
A read-write new-line separated values file which exists on |
|
all cgroups. |
|
|
|
When read, it lists the TIDs of all threads which belong to |
|
the cgroup one-per-line. The TIDs are not ordered and the |
|
same TID may show up more than once if the thread got moved to |
|
another cgroup and then back or the TID got recycled while |
|
reading. |
|
|
|
A TID can be written to migrate the thread associated with the |
|
TID to the cgroup. The writer should match all of the |
|
following conditions. |
|
|
|
- It must have write access to the "cgroup.threads" file. |
|
|
|
- The cgroup that the thread is currently in must be in the |
|
same resource domain as the destination cgroup. |
|
|
|
- It must have write access to the "cgroup.procs" file of the |
|
common ancestor of the source and destination cgroups. |
|
|
|
When delegating a sub-hierarchy, write access to this file |
|
should be granted along with the containing directory. |
|
|
|
cgroup.controllers |
|
A read-only space separated values file which exists on all |
|
cgroups. |
|
|
|
It shows space separated list of all controllers available to |
|
the cgroup. The controllers are not ordered. |
|
|
|
cgroup.subtree_control |
|
A read-write space separated values file which exists on all |
|
cgroups. Starts out empty. |
|
|
|
When read, it shows space separated list of the controllers |
|
which are enabled to control resource distribution from the |
|
cgroup to its children. |
|
|
|
Space separated list of controllers prefixed with '+' or '-' |
|
can be written to enable or disable controllers. A controller |
|
name prefixed with '+' enables the controller and '-' |
|
disables. If a controller appears more than once on the list, |
|
the last one is effective. When multiple enable and disable |
|
operations are specified, either all succeed or all fail. |
|
|
|
cgroup.events |
|
A read-only flat-keyed file which exists on non-root cgroups. |
|
The following entries are defined. Unless specified |
|
otherwise, a value change in this file generates a file |
|
modified event. |
|
|
|
populated |
|
1 if the cgroup or its descendants contains any live |
|
processes; otherwise, 0. |
|
frozen |
|
1 if the cgroup is frozen; otherwise, 0. |
|
|
|
cgroup.max.descendants |
|
A read-write single value files. The default is "max". |
|
|
|
Maximum allowed number of descent cgroups. |
|
If the actual number of descendants is equal or larger, |
|
an attempt to create a new cgroup in the hierarchy will fail. |
|
|
|
cgroup.max.depth |
|
A read-write single value files. The default is "max". |
|
|
|
Maximum allowed descent depth below the current cgroup. |
|
If the actual descent depth is equal or larger, |
|
an attempt to create a new child cgroup will fail. |
|
|
|
cgroup.stat |
|
A read-only flat-keyed file with the following entries: |
|
|
|
nr_descendants |
|
Total number of visible descendant cgroups. |
|
|
|
nr_dying_descendants |
|
Total number of dying descendant cgroups. A cgroup becomes |
|
dying after being deleted by a user. The cgroup will remain |
|
in dying state for some time undefined time (which can depend |
|
on system load) before being completely destroyed. |
|
|
|
A process can't enter a dying cgroup under any circumstances, |
|
a dying cgroup can't revive. |
|
|
|
A dying cgroup can consume system resources not exceeding |
|
limits, which were active at the moment of cgroup deletion. |
|
|
|
cgroup.freeze |
|
A read-write single value file which exists on non-root cgroups. |
|
Allowed values are "0" and "1". The default is "0". |
|
|
|
Writing "1" to the file causes freezing of the cgroup and all |
|
descendant cgroups. This means that all belonging processes will |
|
be stopped and will not run until the cgroup will be explicitly |
|
unfrozen. Freezing of the cgroup may take some time; when this action |
|
is completed, the "frozen" value in the cgroup.events control file |
|
will be updated to "1" and the corresponding notification will be |
|
issued. |
|
|
|
A cgroup can be frozen either by its own settings, or by settings |
|
of any ancestor cgroups. If any of ancestor cgroups is frozen, the |
|
cgroup will remain frozen. |
|
|
|
Processes in the frozen cgroup can be killed by a fatal signal. |
|
They also can enter and leave a frozen cgroup: either by an explicit |
|
move by a user, or if freezing of the cgroup races with fork(). |
|
If a process is moved to a frozen cgroup, it stops. If a process is |
|
moved out of a frozen cgroup, it becomes running. |
|
|
|
Frozen status of a cgroup doesn't affect any cgroup tree operations: |
|
it's possible to delete a frozen (and empty) cgroup, as well as |
|
create new sub-cgroups. |
|
|
|
cgroup.kill |
|
A write-only single value file which exists in non-root cgroups. |
|
The only allowed value is "1". |
|
|
|
Writing "1" to the file causes the cgroup and all descendant cgroups to |
|
be killed. This means that all processes located in the affected cgroup |
|
tree will be killed via SIGKILL. |
|
|
|
Killing a cgroup tree will deal with concurrent forks appropriately and |
|
is protected against migrations. |
|
|
|
In a threaded cgroup, writing this file fails with EOPNOTSUPP as |
|
killing cgroups is a process directed operation, i.e. it affects |
|
the whole thread-group. |
|
|
|
Controllers |
|
=========== |
|
|
|
.. _cgroup-v2-cpu: |
|
|
|
CPU |
|
--- |
|
|
|
The "cpu" controllers regulates distribution of CPU cycles. This |
|
controller implements weight and absolute bandwidth limit models for |
|
normal scheduling policy and absolute bandwidth allocation model for |
|
realtime scheduling policy. |
|
|
|
In all the above models, cycles distribution is defined only on a temporal |
|
base and it does not account for the frequency at which tasks are executed. |
|
The (optional) utilization clamping support allows to hint the schedutil |
|
cpufreq governor about the minimum desired frequency which should always be |
|
provided by a CPU, as well as the maximum desired frequency, which should not |
|
be exceeded by a CPU. |
|
|
|
WARNING: cgroup2 doesn't yet support control of realtime processes and |
|
the cpu controller can only be enabled when all RT processes are in |
|
the root cgroup. Be aware that system management software may already |
|
have placed RT processes into nonroot cgroups during the system boot |
|
process, and these processes may need to be moved to the root cgroup |
|
before the cpu controller can be enabled. |
|
|
|
|
|
CPU Interface Files |
|
~~~~~~~~~~~~~~~~~~~ |
|
|
|
All time durations are in microseconds. |
|
|
|
cpu.stat |
|
A read-only flat-keyed file. |
|
This file exists whether the controller is enabled or not. |
|
|
|
It always reports the following three stats: |
|
|
|
- usage_usec |
|
- user_usec |
|
- system_usec |
|
|
|
and the following three when the controller is enabled: |
|
|
|
- nr_periods |
|
- nr_throttled |
|
- throttled_usec |
|
|
|
cpu.weight |
|
A read-write single value file which exists on non-root |
|
cgroups. The default is "100". |
|
|
|
The weight in the range [1, 10000]. |
|
|
|
cpu.weight.nice |
|
A read-write single value file which exists on non-root |
|
cgroups. The default is "0". |
|
|
|
The nice value is in the range [-20, 19]. |
|
|
|
This interface file is an alternative interface for |
|
"cpu.weight" and allows reading and setting weight using the |
|
same values used by nice(2). Because the range is smaller and |
|
granularity is coarser for the nice values, the read value is |
|
the closest approximation of the current weight. |
|
|
|
cpu.max |
|
A read-write two value file which exists on non-root cgroups. |
|
The default is "max 100000". |
|
|
|
The maximum bandwidth limit. It's in the following format:: |
|
|
|
$MAX $PERIOD |
|
|
|
which indicates that the group may consume upto $MAX in each |
|
$PERIOD duration. "max" for $MAX indicates no limit. If only |
|
one number is written, $MAX is updated. |
|
|
|
cpu.pressure |
|
A read-write nested-keyed file. |
|
|
|
Shows pressure stall information for CPU. See |
|
:ref:`Documentation/accounting/psi.rst <psi>` for details. |
|
|
|
cpu.uclamp.min |
|
A read-write single value file which exists on non-root cgroups. |
|
The default is "0", i.e. no utilization boosting. |
|
|
|
The requested minimum utilization (protection) as a percentage |
|
rational number, e.g. 12.34 for 12.34%. |
|
|
|
This interface allows reading and setting minimum utilization clamp |
|
values similar to the sched_setattr(2). This minimum utilization |
|
value is used to clamp the task specific minimum utilization clamp. |
|
|
|
The requested minimum utilization (protection) is always capped by |
|
the current value for the maximum utilization (limit), i.e. |
|
`cpu.uclamp.max`. |
|
|
|
cpu.uclamp.max |
|
A read-write single value file which exists on non-root cgroups. |
|
The default is "max". i.e. no utilization capping |
|
|
|
The requested maximum utilization (limit) as a percentage rational |
|
number, e.g. 98.76 for 98.76%. |
|
|
|
This interface allows reading and setting maximum utilization clamp |
|
values similar to the sched_setattr(2). This maximum utilization |
|
value is used to clamp the task specific maximum utilization clamp. |
|
|
|
|
|
|
|
Memory |
|
------ |
|
|
|
The "memory" controller regulates distribution of memory. Memory is |
|
stateful and implements both limit and protection models. Due to the |
|
intertwining between memory usage and reclaim pressure and the |
|
stateful nature of memory, the distribution model is relatively |
|
complex. |
|
|
|
While not completely water-tight, all major memory usages by a given |
|
cgroup are tracked so that the total memory consumption can be |
|
accounted and controlled to a reasonable extent. Currently, the |
|
following types of memory usages are tracked. |
|
|
|
- Userland memory - page cache and anonymous memory. |
|
|
|
- Kernel data structures such as dentries and inodes. |
|
|
|
- TCP socket buffers. |
|
|
|
The above list may expand in the future for better coverage. |
|
|
|
|
|
Memory Interface Files |
|
~~~~~~~~~~~~~~~~~~~~~~ |
|
|
|
All memory amounts are in bytes. If a value which is not aligned to |
|
PAGE_SIZE is written, the value may be rounded up to the closest |
|
PAGE_SIZE multiple when read back. |
|
|
|
memory.current |
|
A read-only single value file which exists on non-root |
|
cgroups. |
|
|
|
The total amount of memory currently being used by the cgroup |
|
and its descendants. |
|
|
|
memory.min |
|
A read-write single value file which exists on non-root |
|
cgroups. The default is "0". |
|
|
|
Hard memory protection. If the memory usage of a cgroup |
|
is within its effective min boundary, the cgroup's memory |
|
won't be reclaimed under any conditions. If there is no |
|
unprotected reclaimable memory available, OOM killer |
|
is invoked. Above the effective min boundary (or |
|
effective low boundary if it is higher), pages are reclaimed |
|
proportionally to the overage, reducing reclaim pressure for |
|
smaller overages. |
|
|
|
Effective min boundary is limited by memory.min values of |
|
all ancestor cgroups. If there is memory.min overcommitment |
|
(child cgroup or cgroups are requiring more protected memory |
|
than parent will allow), then each child cgroup will get |
|
the part of parent's protection proportional to its |
|
actual memory usage below memory.min. |
|
|
|
Putting more memory than generally available under this |
|
protection is discouraged and may lead to constant OOMs. |
|
|
|
If a memory cgroup is not populated with processes, |
|
its memory.min is ignored. |
|
|
|
memory.low |
|
A read-write single value file which exists on non-root |
|
cgroups. The default is "0". |
|
|
|
Best-effort memory protection. If the memory usage of a |
|
cgroup is within its effective low boundary, the cgroup's |
|
memory won't be reclaimed unless there is no reclaimable |
|
memory available in unprotected cgroups. |
|
Above the effective low boundary (or |
|
effective min boundary if it is higher), pages are reclaimed |
|
proportionally to the overage, reducing reclaim pressure for |
|
smaller overages. |
|
|
|
Effective low boundary is limited by memory.low values of |
|
all ancestor cgroups. If there is memory.low overcommitment |
|
(child cgroup or cgroups are requiring more protected memory |
|
than parent will allow), then each child cgroup will get |
|
the part of parent's protection proportional to its |
|
actual memory usage below memory.low. |
|
|
|
Putting more memory than generally available under this |
|
protection is discouraged. |
|
|
|
memory.high |
|
A read-write single value file which exists on non-root |
|
cgroups. The default is "max". |
|
|
|
Memory usage throttle limit. This is the main mechanism to |
|
control memory usage of a cgroup. If a cgroup's usage goes |
|
over the high boundary, the processes of the cgroup are |
|
throttled and put under heavy reclaim pressure. |
|
|
|
Going over the high limit never invokes the OOM killer and |
|
under extreme conditions the limit may be breached. |
|
|
|
memory.max |
|
A read-write single value file which exists on non-root |
|
cgroups. The default is "max". |
|
|
|
Memory usage hard limit. This is the final protection |
|
mechanism. If a cgroup's memory usage reaches this limit and |
|
can't be reduced, the OOM killer is invoked in the cgroup. |
|
Under certain circumstances, the usage may go over the limit |
|
temporarily. |
|
|
|
In default configuration regular 0-order allocations always |
|
succeed unless OOM killer chooses current task as a victim. |
|
|
|
Some kinds of allocations don't invoke the OOM killer. |
|
Caller could retry them differently, return into userspace |
|
as -ENOMEM or silently ignore in cases like disk readahead. |
|
|
|
This is the ultimate protection mechanism. As long as the |
|
high limit is used and monitored properly, this limit's |
|
utility is limited to providing the final safety net. |
|
|
|
memory.oom.group |
|
A read-write single value file which exists on non-root |
|
cgroups. The default value is "0". |
|
|
|
Determines whether the cgroup should be treated as |
|
an indivisible workload by the OOM killer. If set, |
|
all tasks belonging to the cgroup or to its descendants |
|
(if the memory cgroup is not a leaf cgroup) are killed |
|
together or not at all. This can be used to avoid |
|
partial kills to guarantee workload integrity. |
|
|
|
Tasks with the OOM protection (oom_score_adj set to -1000) |
|
are treated as an exception and are never killed. |
|
|
|
If the OOM killer is invoked in a cgroup, it's not going |
|
to kill any tasks outside of this cgroup, regardless |
|
memory.oom.group values of ancestor cgroups. |
|
|
|
memory.events |
|
A read-only flat-keyed file which exists on non-root cgroups. |
|
The following entries are defined. Unless specified |
|
otherwise, a value change in this file generates a file |
|
modified event. |
|
|
|
Note that all fields in this file are hierarchical and the |
|
file modified event can be generated due to an event down the |
|
hierarchy. For for the local events at the cgroup level see |
|
memory.events.local. |
|
|
|
low |
|
The number of times the cgroup is reclaimed due to |
|
high memory pressure even though its usage is under |
|
the low boundary. This usually indicates that the low |
|
boundary is over-committed. |
|
|
|
high |
|
The number of times processes of the cgroup are |
|
throttled and routed to perform direct memory reclaim |
|
because the high memory boundary was exceeded. For a |
|
cgroup whose memory usage is capped by the high limit |
|
rather than global memory pressure, this event's |
|
occurrences are expected. |
|
|
|
max |
|
The number of times the cgroup's memory usage was |
|
about to go over the max boundary. If direct reclaim |
|
fails to bring it down, the cgroup goes to OOM state. |
|
|
|
oom |
|
The number of time the cgroup's memory usage was |
|
reached the limit and allocation was about to fail. |
|
|
|
This event is not raised if the OOM killer is not |
|
considered as an option, e.g. for failed high-order |
|
allocations or if caller asked to not retry attempts. |
|
|
|
oom_kill |
|
The number of processes belonging to this cgroup |
|
killed by any kind of OOM killer. |
|
|
|
memory.events.local |
|
Similar to memory.events but the fields in the file are local |
|
to the cgroup i.e. not hierarchical. The file modified event |
|
generated on this file reflects only the local events. |
|
|
|
memory.stat |
|
A read-only flat-keyed file which exists on non-root cgroups. |
|
|
|
This breaks down the cgroup's memory footprint into different |
|
types of memory, type-specific details, and other information |
|
on the state and past events of the memory management system. |
|
|
|
All memory amounts are in bytes. |
|
|
|
The entries are ordered to be human readable, and new entries |
|
can show up in the middle. Don't rely on items remaining in a |
|
fixed position; use the keys to look up specific values! |
|
|
|
If the entry has no per-node counter (or not show in the |
|
memory.numa_stat). We use 'npn' (non-per-node) as the tag |
|
to indicate that it will not show in the memory.numa_stat. |
|
|
|
anon |
|
Amount of memory used in anonymous mappings such as |
|
brk(), sbrk(), and mmap(MAP_ANONYMOUS) |
|
|
|
file |
|
Amount of memory used to cache filesystem data, |
|
including tmpfs and shared memory. |
|
|
|
kernel_stack |
|
Amount of memory allocated to kernel stacks. |
|
|
|
pagetables |
|
Amount of memory allocated for page tables. |
|
|
|
percpu (npn) |
|
Amount of memory used for storing per-cpu kernel |
|
data structures. |
|
|
|
sock (npn) |
|
Amount of memory used in network transmission buffers |
|
|
|
shmem |
|
Amount of cached filesystem data that is swap-backed, |
|
such as tmpfs, shm segments, shared anonymous mmap()s |
|
|
|
file_mapped |
|
Amount of cached filesystem data mapped with mmap() |
|
|
|
file_dirty |
|
Amount of cached filesystem data that was modified but |
|
not yet written back to disk |
|
|
|
file_writeback |
|
Amount of cached filesystem data that was modified and |
|
is currently being written back to disk |
|
|
|
swapcached |
|
Amount of swap cached in memory. The swapcache is accounted |
|
against both memory and swap usage. |
|
|
|
anon_thp |
|
Amount of memory used in anonymous mappings backed by |
|
transparent hugepages |
|
|
|
file_thp |
|
Amount of cached filesystem data backed by transparent |
|
hugepages |
|
|
|
shmem_thp |
|
Amount of shm, tmpfs, shared anonymous mmap()s backed by |
|
transparent hugepages |
|
|
|
inactive_anon, active_anon, inactive_file, active_file, unevictable |
|
Amount of memory, swap-backed and filesystem-backed, |
|
on the internal memory management lists used by the |
|
page reclaim algorithm. |
|
|
|
As these represent internal list state (eg. shmem pages are on anon |
|
memory management lists), inactive_foo + active_foo may not be equal to |
|
the value for the foo counter, since the foo counter is type-based, not |
|
list-based. |
|
|
|
slab_reclaimable |
|
Part of "slab" that might be reclaimed, such as |
|
dentries and inodes. |
|
|
|
slab_unreclaimable |
|
Part of "slab" that cannot be reclaimed on memory |
|
pressure. |
|
|
|
slab (npn) |
|
Amount of memory used for storing in-kernel data |
|
structures. |
|
|
|
workingset_refault_anon |
|
Number of refaults of previously evicted anonymous pages. |
|
|
|
workingset_refault_file |
|
Number of refaults of previously evicted file pages. |
|
|
|
workingset_activate_anon |
|
Number of refaulted anonymous pages that were immediately |
|
activated. |
|
|
|
workingset_activate_file |
|
Number of refaulted file pages that were immediately activated. |
|
|
|
workingset_restore_anon |
|
Number of restored anonymous pages which have been detected as |
|
an active workingset before they got reclaimed. |
|
|
|
workingset_restore_file |
|
Number of restored file pages which have been detected as an |
|
active workingset before they got reclaimed. |
|
|
|
workingset_nodereclaim |
|
Number of times a shadow node has been reclaimed |
|
|
|
pgfault (npn) |
|
Total number of page faults incurred |
|
|
|
pgmajfault (npn) |
|
Number of major page faults incurred |
|
|
|
pgrefill (npn) |
|
Amount of scanned pages (in an active LRU list) |
|
|
|
pgscan (npn) |
|
Amount of scanned pages (in an inactive LRU list) |
|
|
|
pgsteal (npn) |
|
Amount of reclaimed pages |
|
|
|
pgactivate (npn) |
|
Amount of pages moved to the active LRU list |
|
|
|
pgdeactivate (npn) |
|
Amount of pages moved to the inactive LRU list |
|
|
|
pglazyfree (npn) |
|
Amount of pages postponed to be freed under memory pressure |
|
|
|
pglazyfreed (npn) |
|
Amount of reclaimed lazyfree pages |
|
|
|
thp_fault_alloc (npn) |
|
Number of transparent hugepages which were allocated to satisfy |
|
a page fault. This counter is not present when CONFIG_TRANSPARENT_HUGEPAGE |
|
is not set. |
|
|
|
thp_collapse_alloc (npn) |
|
Number of transparent hugepages which were allocated to allow |
|
collapsing an existing range of pages. This counter is not |
|
present when CONFIG_TRANSPARENT_HUGEPAGE is not set. |
|
|
|
memory.numa_stat |
|
A read-only nested-keyed file which exists on non-root cgroups. |
|
|
|
This breaks down the cgroup's memory footprint into different |
|
types of memory, type-specific details, and other information |
|
per node on the state of the memory management system. |
|
|
|
This is useful for providing visibility into the NUMA locality |
|
information within an memcg since the pages are allowed to be |
|
allocated from any physical node. One of the use case is evaluating |
|
application performance by combining this information with the |
|
application's CPU allocation. |
|
|
|
All memory amounts are in bytes. |
|
|
|
The output format of memory.numa_stat is:: |
|
|
|
type N0=<bytes in node 0> N1=<bytes in node 1> ... |
|
|
|
The entries are ordered to be human readable, and new entries |
|
can show up in the middle. Don't rely on items remaining in a |
|
fixed position; use the keys to look up specific values! |
|
|
|
The entries can refer to the memory.stat. |
|
|
|
memory.swap.current |
|
A read-only single value file which exists on non-root |
|
cgroups. |
|
|
|
The total amount of swap currently being used by the cgroup |
|
and its descendants. |
|
|
|
memory.swap.high |
|
A read-write single value file which exists on non-root |
|
cgroups. The default is "max". |
|
|
|
Swap usage throttle limit. If a cgroup's swap usage exceeds |
|
this limit, all its further allocations will be throttled to |
|
allow userspace to implement custom out-of-memory procedures. |
|
|
|
This limit marks a point of no return for the cgroup. It is NOT |
|
designed to manage the amount of swapping a workload does |
|
during regular operation. Compare to memory.swap.max, which |
|
prohibits swapping past a set amount, but lets the cgroup |
|
continue unimpeded as long as other memory can be reclaimed. |
|
|
|
Healthy workloads are not expected to reach this limit. |
|
|
|
memory.swap.max |
|
A read-write single value file which exists on non-root |
|
cgroups. The default is "max". |
|
|
|
Swap usage hard limit. If a cgroup's swap usage reaches this |
|
limit, anonymous memory of the cgroup will not be swapped out. |
|
|
|
memory.swap.events |
|
A read-only flat-keyed file which exists on non-root cgroups. |
|
The following entries are defined. Unless specified |
|
otherwise, a value change in this file generates a file |
|
modified event. |
|
|
|
high |
|
The number of times the cgroup's swap usage was over |
|
the high threshold. |
|
|
|
max |
|
The number of times the cgroup's swap usage was about |
|
to go over the max boundary and swap allocation |
|
failed. |
|
|
|
fail |
|
The number of times swap allocation failed either |
|
because of running out of swap system-wide or max |
|
limit. |
|
|
|
When reduced under the current usage, the existing swap |
|
entries are reclaimed gradually and the swap usage may stay |
|
higher than the limit for an extended period of time. This |
|
reduces the impact on the workload and memory management. |
|
|
|
memory.pressure |
|
A read-only nested-keyed file. |
|
|
|
Shows pressure stall information for memory. See |
|
:ref:`Documentation/accounting/psi.rst <psi>` for details. |
|
|
|
|
|
Usage Guidelines |
|
~~~~~~~~~~~~~~~~ |
|
|
|
"memory.high" is the main mechanism to control memory usage. |
|
Over-committing on high limit (sum of high limits > available memory) |
|
and letting global memory pressure to distribute memory according to |
|
usage is a viable strategy. |
|
|
|
Because breach of the high limit doesn't trigger the OOM killer but |
|
throttles the offending cgroup, a management agent has ample |
|
opportunities to monitor and take appropriate actions such as granting |
|
more memory or terminating the workload. |
|
|
|
Determining whether a cgroup has enough memory is not trivial as |
|
memory usage doesn't indicate whether the workload can benefit from |
|
more memory. For example, a workload which writes data received from |
|
network to a file can use all available memory but can also operate as |
|
performant with a small amount of memory. A measure of memory |
|
pressure - how much the workload is being impacted due to lack of |
|
memory - is necessary to determine whether a workload needs more |
|
memory; unfortunately, memory pressure monitoring mechanism isn't |
|
implemented yet. |
|
|
|
|
|
Memory Ownership |
|
~~~~~~~~~~~~~~~~ |
|
|
|
A memory area is charged to the cgroup which instantiated it and stays |
|
charged to the cgroup until the area is released. Migrating a process |
|
to a different cgroup doesn't move the memory usages that it |
|
instantiated while in the previous cgroup to the new cgroup. |
|
|
|
A memory area may be used by processes belonging to different cgroups. |
|
To which cgroup the area will be charged is in-deterministic; however, |
|
over time, the memory area is likely to end up in a cgroup which has |
|
enough memory allowance to avoid high reclaim pressure. |
|
|
|
If a cgroup sweeps a considerable amount of memory which is expected |
|
to be accessed repeatedly by other cgroups, it may make sense to use |
|
POSIX_FADV_DONTNEED to relinquish the ownership of memory areas |
|
belonging to the affected files to ensure correct memory ownership. |
|
|
|
|
|
IO |
|
-- |
|
|
|
The "io" controller regulates the distribution of IO resources. This |
|
controller implements both weight based and absolute bandwidth or IOPS |
|
limit distribution; however, weight based distribution is available |
|
only if cfq-iosched is in use and neither scheme is available for |
|
blk-mq devices. |
|
|
|
|
|
IO Interface Files |
|
~~~~~~~~~~~~~~~~~~ |
|
|
|
io.stat |
|
A read-only nested-keyed file. |
|
|
|
Lines are keyed by $MAJ:$MIN device numbers and not ordered. |
|
The following nested keys are defined. |
|
|
|
====== ===================== |
|
rbytes Bytes read |
|
wbytes Bytes written |
|
rios Number of read IOs |
|
wios Number of write IOs |
|
dbytes Bytes discarded |
|
dios Number of discard IOs |
|
====== ===================== |
|
|
|
An example read output follows:: |
|
|
|
8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353 dbytes=0 dios=0 |
|
8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252 dbytes=50331648 dios=3021 |
|
|
|
io.cost.qos |
|
A read-write nested-keyed file which exists only on the root |
|
cgroup. |
|
|
|
This file configures the Quality of Service of the IO cost |
|
model based controller (CONFIG_BLK_CGROUP_IOCOST) which |
|
currently implements "io.weight" proportional control. Lines |
|
are keyed by $MAJ:$MIN device numbers and not ordered. The |
|
line for a given device is populated on the first write for |
|
the device on "io.cost.qos" or "io.cost.model". The following |
|
nested keys are defined. |
|
|
|
====== ===================================== |
|
enable Weight-based control enable |
|
ctrl "auto" or "user" |
|
rpct Read latency percentile [0, 100] |
|
rlat Read latency threshold |
|
wpct Write latency percentile [0, 100] |
|
wlat Write latency threshold |
|
min Minimum scaling percentage [1, 10000] |
|
max Maximum scaling percentage [1, 10000] |
|
====== ===================================== |
|
|
|
The controller is disabled by default and can be enabled by |
|
setting "enable" to 1. "rpct" and "wpct" parameters default |
|
to zero and the controller uses internal device saturation |
|
state to adjust the overall IO rate between "min" and "max". |
|
|
|
When a better control quality is needed, latency QoS |
|
parameters can be configured. For example:: |
|
|
|
8:16 enable=1 ctrl=auto rpct=95.00 rlat=75000 wpct=95.00 wlat=150000 min=50.00 max=150.0 |
|
|
|
shows that on sdb, the controller is enabled, will consider |
|
the device saturated if the 95th percentile of read completion |
|
latencies is above 75ms or write 150ms, and adjust the overall |
|
IO issue rate between 50% and 150% accordingly. |
|
|
|
The lower the saturation point, the better the latency QoS at |
|
the cost of aggregate bandwidth. The narrower the allowed |
|
adjustment range between "min" and "max", the more conformant |
|
to the cost model the IO behavior. Note that the IO issue |
|
base rate may be far off from 100% and setting "min" and "max" |
|
blindly can lead to a significant loss of device capacity or |
|
control quality. "min" and "max" are useful for regulating |
|
devices which show wide temporary behavior changes - e.g. a |
|
ssd which accepts writes at the line speed for a while and |
|
then completely stalls for multiple seconds. |
|
|
|
When "ctrl" is "auto", the parameters are controlled by the |
|
kernel and may change automatically. Setting "ctrl" to "user" |
|
or setting any of the percentile and latency parameters puts |
|
it into "user" mode and disables the automatic changes. The |
|
automatic mode can be restored by setting "ctrl" to "auto". |
|
|
|
io.cost.model |
|
A read-write nested-keyed file which exists only on the root |
|
cgroup. |
|
|
|
This file configures the cost model of the IO cost model based |
|
controller (CONFIG_BLK_CGROUP_IOCOST) which currently |
|
implements "io.weight" proportional control. Lines are keyed |
|
by $MAJ:$MIN device numbers and not ordered. The line for a |
|
given device is populated on the first write for the device on |
|
"io.cost.qos" or "io.cost.model". The following nested keys |
|
are defined. |
|
|
|
===== ================================ |
|
ctrl "auto" or "user" |
|
model The cost model in use - "linear" |
|
===== ================================ |
|
|
|
When "ctrl" is "auto", the kernel may change all parameters |
|
dynamically. When "ctrl" is set to "user" or any other |
|
parameters are written to, "ctrl" become "user" and the |
|
automatic changes are disabled. |
|
|
|
When "model" is "linear", the following model parameters are |
|
defined. |
|
|
|
============= ======================================== |
|
[r|w]bps The maximum sequential IO throughput |
|
[r|w]seqiops The maximum 4k sequential IOs per second |
|
[r|w]randiops The maximum 4k random IOs per second |
|
============= ======================================== |
|
|
|
From the above, the builtin linear model determines the base |
|
costs of a sequential and random IO and the cost coefficient |
|
for the IO size. While simple, this model can cover most |
|
common device classes acceptably. |
|
|
|
The IO cost model isn't expected to be accurate in absolute |
|
sense and is scaled to the device behavior dynamically. |
|
|
|
If needed, tools/cgroup/iocost_coef_gen.py can be used to |
|
generate device-specific coefficients. |
|
|
|
io.weight |
|
A read-write flat-keyed file which exists on non-root cgroups. |
|
The default is "default 100". |
|
|
|
The first line is the default weight applied to devices |
|
without specific override. The rest are overrides keyed by |
|
$MAJ:$MIN device numbers and not ordered. The weights are in |
|
the range [1, 10000] and specifies the relative amount IO time |
|
the cgroup can use in relation to its siblings. |
|
|
|
The default weight can be updated by writing either "default |
|
$WEIGHT" or simply "$WEIGHT". Overrides can be set by writing |
|
"$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default". |
|
|
|
An example read output follows:: |
|
|
|
default 100 |
|
8:16 200 |
|
8:0 50 |
|
|
|
io.max |
|
A read-write nested-keyed file which exists on non-root |
|
cgroups. |
|
|
|
BPS and IOPS based IO limit. Lines are keyed by $MAJ:$MIN |
|
device numbers and not ordered. The following nested keys are |
|
defined. |
|
|
|
===== ================================== |
|
rbps Max read bytes per second |
|
wbps Max write bytes per second |
|
riops Max read IO operations per second |
|
wiops Max write IO operations per second |
|
===== ================================== |
|
|
|
When writing, any number of nested key-value pairs can be |
|
specified in any order. "max" can be specified as the value |
|
to remove a specific limit. If the same key is specified |
|
multiple times, the outcome is undefined. |
|
|
|
BPS and IOPS are measured in each IO direction and IOs are |
|
delayed if limit is reached. Temporary bursts are allowed. |
|
|
|
Setting read limit at 2M BPS and write at 120 IOPS for 8:16:: |
|
|
|
echo "8:16 rbps=2097152 wiops=120" > io.max |
|
|
|
Reading returns the following:: |
|
|
|
8:16 rbps=2097152 wbps=max riops=max wiops=120 |
|
|
|
Write IOPS limit can be removed by writing the following:: |
|
|
|
echo "8:16 wiops=max" > io.max |
|
|
|
Reading now returns the following:: |
|
|
|
8:16 rbps=2097152 wbps=max riops=max wiops=max |
|
|
|
io.pressure |
|
A read-only nested-keyed file. |
|
|
|
Shows pressure stall information for IO. See |
|
:ref:`Documentation/accounting/psi.rst <psi>` for details. |
|
|
|
|
|
Writeback |
|
~~~~~~~~~ |
|
|
|
Page cache is dirtied through buffered writes and shared mmaps and |
|
written asynchronously to the backing filesystem by the writeback |
|
mechanism. Writeback sits between the memory and IO domains and |
|
regulates the proportion of dirty memory by balancing dirtying and |
|
write IOs. |
|
|
|
The io controller, in conjunction with the memory controller, |
|
implements control of page cache writeback IOs. The memory controller |
|
defines the memory domain that dirty memory ratio is calculated and |
|
maintained for and the io controller defines the io domain which |
|
writes out dirty pages for the memory domain. Both system-wide and |
|
per-cgroup dirty memory states are examined and the more restrictive |
|
of the two is enforced. |
|
|
|
cgroup writeback requires explicit support from the underlying |
|
filesystem. Currently, cgroup writeback is implemented on ext2, ext4, |
|
btrfs, f2fs, and xfs. On other filesystems, all writeback IOs are |
|
attributed to the root cgroup. |
|
|
|
There are inherent differences in memory and writeback management |
|
which affects how cgroup ownership is tracked. Memory is tracked per |
|
page while writeback per inode. For the purpose of writeback, an |
|
inode is assigned to a cgroup and all IO requests to write dirty pages |
|
from the inode are attributed to that cgroup. |
|
|
|
As cgroup ownership for memory is tracked per page, there can be pages |
|
which are associated with different cgroups than the one the inode is |
|
associated with. These are called foreign pages. The writeback |
|
constantly keeps track of foreign pages and, if a particular foreign |
|
cgroup becomes the majority over a certain period of time, switches |
|
the ownership of the inode to that cgroup. |
|
|
|
While this model is enough for most use cases where a given inode is |
|
mostly dirtied by a single cgroup even when the main writing cgroup |
|
changes over time, use cases where multiple cgroups write to a single |
|
inode simultaneously are not supported well. In such circumstances, a |
|
significant portion of IOs are likely to be attributed incorrectly. |
|
As memory controller assigns page ownership on the first use and |
|
doesn't update it until the page is released, even if writeback |
|
strictly follows page ownership, multiple cgroups dirtying overlapping |
|
areas wouldn't work as expected. It's recommended to avoid such usage |
|
patterns. |
|
|
|
The sysctl knobs which affect writeback behavior are applied to cgroup |
|
writeback as follows. |
|
|
|
vm.dirty_background_ratio, vm.dirty_ratio |
|
These ratios apply the same to cgroup writeback with the |
|
amount of available memory capped by limits imposed by the |
|
memory controller and system-wide clean memory. |
|
|
|
vm.dirty_background_bytes, vm.dirty_bytes |
|
For cgroup writeback, this is calculated into ratio against |
|
total available memory and applied the same way as |
|
vm.dirty[_background]_ratio. |
|
|
|
|
|
IO Latency |
|
~~~~~~~~~~ |
|
|
|
This is a cgroup v2 controller for IO workload protection. You provide a group |
|
with a latency target, and if the average latency exceeds that target the |
|
controller will throttle any peers that have a lower latency target than the |
|
protected workload. |
|
|
|
The limits are only applied at the peer level in the hierarchy. This means that |
|
in the diagram below, only groups A, B, and C will influence each other, and |
|
groups D and F will influence each other. Group G will influence nobody:: |
|
|
|
[root] |
|
/ | \ |
|
A B C |
|
/ \ | |
|
D F G |
|
|
|
|
|
So the ideal way to configure this is to set io.latency in groups A, B, and C. |
|
Generally you do not want to set a value lower than the latency your device |
|
supports. Experiment to find the value that works best for your workload. |
|
Start at higher than the expected latency for your device and watch the |
|
avg_lat value in io.stat for your workload group to get an idea of the |
|
latency you see during normal operation. Use the avg_lat value as a basis for |
|
your real setting, setting at 10-15% higher than the value in io.stat. |
|
|
|
How IO Latency Throttling Works |
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
|
|
|
io.latency is work conserving; so as long as everybody is meeting their latency |
|
target the controller doesn't do anything. Once a group starts missing its |
|
target it begins throttling any peer group that has a higher target than itself. |
|
This throttling takes 2 forms: |
|
|
|
- Queue depth throttling. This is the number of outstanding IO's a group is |
|
allowed to have. We will clamp down relatively quickly, starting at no limit |
|
and going all the way down to 1 IO at a time. |
|
|
|
- Artificial delay induction. There are certain types of IO that cannot be |
|
throttled without possibly adversely affecting higher priority groups. This |
|
includes swapping and metadata IO. These types of IO are allowed to occur |
|
normally, however they are "charged" to the originating group. If the |
|
originating group is being throttled you will see the use_delay and delay |
|
fields in io.stat increase. The delay value is how many microseconds that are |
|
being added to any process that runs in this group. Because this number can |
|
grow quite large if there is a lot of swapping or metadata IO occurring we |
|
limit the individual delay events to 1 second at a time. |
|
|
|
Once the victimized group starts meeting its latency target again it will start |
|
unthrottling any peer groups that were throttled previously. If the victimized |
|
group simply stops doing IO the global counter will unthrottle appropriately. |
|
|
|
IO Latency Interface Files |
|
~~~~~~~~~~~~~~~~~~~~~~~~~~ |
|
|
|
io.latency |
|
This takes a similar format as the other controllers. |
|
|
|
"MAJOR:MINOR target=<target time in microseconds" |
|
|
|
io.stat |
|
If the controller is enabled you will see extra stats in io.stat in |
|
addition to the normal ones. |
|
|
|
depth |
|
This is the current queue depth for the group. |
|
|
|
avg_lat |
|
This is an exponential moving average with a decay rate of 1/exp |
|
bound by the sampling interval. The decay rate interval can be |
|
calculated by multiplying the win value in io.stat by the |
|
corresponding number of samples based on the win value. |
|
|
|
win |
|
The sampling window size in milliseconds. This is the minimum |
|
duration of time between evaluation events. Windows only elapse |
|
with IO activity. Idle periods extend the most recent window. |
|
|
|
IO Priority |
|
~~~~~~~~~~~ |
|
|
|
A single attribute controls the behavior of the I/O priority cgroup policy, |
|
namely the blkio.prio.class attribute. The following values are accepted for |
|
that attribute: |
|
|
|
no-change |
|
Do not modify the I/O priority class. |
|
|
|
none-to-rt |
|
For requests that do not have an I/O priority class (NONE), |
|
change the I/O priority class into RT. Do not modify |
|
the I/O priority class of other requests. |
|
|
|
restrict-to-be |
|
For requests that do not have an I/O priority class or that have I/O |
|
priority class RT, change it into BE. Do not modify the I/O priority |
|
class of requests that have priority class IDLE. |
|
|
|
idle |
|
Change the I/O priority class of all requests into IDLE, the lowest |
|
I/O priority class. |
|
|
|
The following numerical values are associated with the I/O priority policies: |
|
|
|
+-------------+---+ |
|
| no-change | 0 | |
|
+-------------+---+ |
|
| none-to-rt | 1 | |
|
+-------------+---+ |
|
| rt-to-be | 2 | |
|
+-------------+---+ |
|
| all-to-idle | 3 | |
|
+-------------+---+ |
|
|
|
The numerical value that corresponds to each I/O priority class is as follows: |
|
|
|
+-------------------------------+---+ |
|
| IOPRIO_CLASS_NONE | 0 | |
|
+-------------------------------+---+ |
|
| IOPRIO_CLASS_RT (real-time) | 1 | |
|
+-------------------------------+---+ |
|
| IOPRIO_CLASS_BE (best effort) | 2 | |
|
+-------------------------------+---+ |
|
| IOPRIO_CLASS_IDLE | 3 | |
|
+-------------------------------+---+ |
|
|
|
The algorithm to set the I/O priority class for a request is as follows: |
|
|
|
- Translate the I/O priority class policy into a number. |
|
- Change the request I/O priority class into the maximum of the I/O priority |
|
class policy number and the numerical I/O priority class. |
|
|
|
PID |
|
--- |
|
|
|
The process number controller is used to allow a cgroup to stop any |
|
new tasks from being fork()'d or clone()'d after a specified limit is |
|
reached. |
|
|
|
The number of tasks in a cgroup can be exhausted in ways which other |
|
controllers cannot prevent, thus warranting its own controller. For |
|
example, a fork bomb is likely to exhaust the number of tasks before |
|
hitting memory restrictions. |
|
|
|
Note that PIDs used in this controller refer to TIDs, process IDs as |
|
used by the kernel. |
|
|
|
|
|
PID Interface Files |
|
~~~~~~~~~~~~~~~~~~~ |
|
|
|
pids.max |
|
A read-write single value file which exists on non-root |
|
cgroups. The default is "max". |
|
|
|
Hard limit of number of processes. |
|
|
|
pids.current |
|
A read-only single value file which exists on all cgroups. |
|
|
|
The number of processes currently in the cgroup and its |
|
descendants. |
|
|
|
Organisational operations are not blocked by cgroup policies, so it is |
|
possible to have pids.current > pids.max. This can be done by either |
|
setting the limit to be smaller than pids.current, or attaching enough |
|
processes to the cgroup such that pids.current is larger than |
|
pids.max. However, it is not possible to violate a cgroup PID policy |
|
through fork() or clone(). These will return -EAGAIN if the creation |
|
of a new process would cause a cgroup policy to be violated. |
|
|
|
|
|
Cpuset |
|
------ |
|
|
|
The "cpuset" controller provides a mechanism for constraining |
|
the CPU and memory node placement of tasks to only the resources |
|
specified in the cpuset interface files in a task's current cgroup. |
|
This is especially valuable on large NUMA systems where placing jobs |
|
on properly sized subsets of the systems with careful processor and |
|
memory placement to reduce cross-node memory access and contention |
|
can improve overall system performance. |
|
|
|
The "cpuset" controller is hierarchical. That means the controller |
|
cannot use CPUs or memory nodes not allowed in its parent. |
|
|
|
|
|
Cpuset Interface Files |
|
~~~~~~~~~~~~~~~~~~~~~~ |
|
|
|
cpuset.cpus |
|
A read-write multiple values file which exists on non-root |
|
cpuset-enabled cgroups. |
|
|
|
It lists the requested CPUs to be used by tasks within this |
|
cgroup. The actual list of CPUs to be granted, however, is |
|
subjected to constraints imposed by its parent and can differ |
|
from the requested CPUs. |
|
|
|
The CPU numbers are comma-separated numbers or ranges. |
|
For example:: |
|
|
|
# cat cpuset.cpus |
|
0-4,6,8-10 |
|
|
|
An empty value indicates that the cgroup is using the same |
|
setting as the nearest cgroup ancestor with a non-empty |
|
"cpuset.cpus" or all the available CPUs if none is found. |
|
|
|
The value of "cpuset.cpus" stays constant until the next update |
|
and won't be affected by any CPU hotplug events. |
|
|
|
cpuset.cpus.effective |
|
A read-only multiple values file which exists on all |
|
cpuset-enabled cgroups. |
|
|
|
It lists the onlined CPUs that are actually granted to this |
|
cgroup by its parent. These CPUs are allowed to be used by |
|
tasks within the current cgroup. |
|
|
|
If "cpuset.cpus" is empty, the "cpuset.cpus.effective" file shows |
|
all the CPUs from the parent cgroup that can be available to |
|
be used by this cgroup. Otherwise, it should be a subset of |
|
"cpuset.cpus" unless none of the CPUs listed in "cpuset.cpus" |
|
can be granted. In this case, it will be treated just like an |
|
empty "cpuset.cpus". |
|
|
|
Its value will be affected by CPU hotplug events. |
|
|
|
cpuset.mems |
|
A read-write multiple values file which exists on non-root |
|
cpuset-enabled cgroups. |
|
|
|
It lists the requested memory nodes to be used by tasks within |
|
this cgroup. The actual list of memory nodes granted, however, |
|
is subjected to constraints imposed by its parent and can differ |
|
from the requested memory nodes. |
|
|
|
The memory node numbers are comma-separated numbers or ranges. |
|
For example:: |
|
|
|
# cat cpuset.mems |
|
0-1,3 |
|
|
|
An empty value indicates that the cgroup is using the same |
|
setting as the nearest cgroup ancestor with a non-empty |
|
"cpuset.mems" or all the available memory nodes if none |
|
is found. |
|
|
|
The value of "cpuset.mems" stays constant until the next update |
|
and won't be affected by any memory nodes hotplug events. |
|
|
|
Setting a non-empty value to "cpuset.mems" causes memory of |
|
tasks within the cgroup to be migrated to the designated nodes if |
|
they are currently using memory outside of the designated nodes. |
|
|
|
There is a cost for this memory migration. The migration |
|
may not be complete and some memory pages may be left behind. |
|
So it is recommended that "cpuset.mems" should be set properly |
|
before spawning new tasks into the cpuset. Even if there is |
|
a need to change "cpuset.mems" with active tasks, it shouldn't |
|
be done frequently. |
|
|
|
cpuset.mems.effective |
|
A read-only multiple values file which exists on all |
|
cpuset-enabled cgroups. |
|
|
|
It lists the onlined memory nodes that are actually granted to |
|
this cgroup by its parent. These memory nodes are allowed to |
|
be used by tasks within the current cgroup. |
|
|
|
If "cpuset.mems" is empty, it shows all the memory nodes from the |
|
parent cgroup that will be available to be used by this cgroup. |
|
Otherwise, it should be a subset of "cpuset.mems" unless none of |
|
the memory nodes listed in "cpuset.mems" can be granted. In this |
|
case, it will be treated just like an empty "cpuset.mems". |
|
|
|
Its value will be affected by memory nodes hotplug events. |
|
|
|
cpuset.cpus.partition |
|
A read-write single value file which exists on non-root |
|
cpuset-enabled cgroups. This flag is owned by the parent cgroup |
|
and is not delegatable. |
|
|
|
It accepts only the following input values when written to. |
|
|
|
======== ================================ |
|
"root" a partition root |
|
"member" a non-root member of a partition |
|
======== ================================ |
|
|
|
When set to be a partition root, the current cgroup is the |
|
root of a new partition or scheduling domain that comprises |
|
itself and all its descendants except those that are separate |
|
partition roots themselves and their descendants. The root |
|
cgroup is always a partition root. |
|
|
|
There are constraints on where a partition root can be set. |
|
It can only be set in a cgroup if all the following conditions |
|
are true. |
|
|
|
1) The "cpuset.cpus" is not empty and the list of CPUs are |
|
exclusive, i.e. they are not shared by any of its siblings. |
|
2) The parent cgroup is a partition root. |
|
3) The "cpuset.cpus" is also a proper subset of the parent's |
|
"cpuset.cpus.effective". |
|
4) There is no child cgroups with cpuset enabled. This is for |
|
eliminating corner cases that have to be handled if such a |
|
condition is allowed. |
|
|
|
Setting it to partition root will take the CPUs away from the |
|
effective CPUs of the parent cgroup. Once it is set, this |
|
file cannot be reverted back to "member" if there are any child |
|
cgroups with cpuset enabled. |
|
|
|
A parent partition cannot distribute all its CPUs to its |
|
child partitions. There must be at least one cpu left in the |
|
parent partition. |
|
|
|
Once becoming a partition root, changes to "cpuset.cpus" is |
|
generally allowed as long as the first condition above is true, |
|
the change will not take away all the CPUs from the parent |
|
partition and the new "cpuset.cpus" value is a superset of its |
|
children's "cpuset.cpus" values. |
|
|
|
Sometimes, external factors like changes to ancestors' |
|
"cpuset.cpus" or cpu hotplug can cause the state of the partition |
|
root to change. On read, the "cpuset.sched.partition" file |
|
can show the following values. |
|
|
|
============== ============================== |
|
"member" Non-root member of a partition |
|
"root" Partition root |
|
"root invalid" Invalid partition root |
|
============== ============================== |
|
|
|
It is a partition root if the first 2 partition root conditions |
|
above are true and at least one CPU from "cpuset.cpus" is |
|
granted by the parent cgroup. |
|
|
|
A partition root can become invalid if none of CPUs requested |
|
in "cpuset.cpus" can be granted by the parent cgroup or the |
|
parent cgroup is no longer a partition root itself. In this |
|
case, it is not a real partition even though the restriction |
|
of the first partition root condition above will still apply. |
|
The cpu affinity of all the tasks in the cgroup will then be |
|
associated with CPUs in the nearest ancestor partition. |
|
|
|
An invalid partition root can be transitioned back to a |
|
real partition root if at least one of the requested CPUs |
|
can now be granted by its parent. In this case, the cpu |
|
affinity of all the tasks in the formerly invalid partition |
|
will be associated to the CPUs of the newly formed partition. |
|
Changing the partition state of an invalid partition root to |
|
"member" is always allowed even if child cpusets are present. |
|
|
|
|
|
Device controller |
|
----------------- |
|
|
|
Device controller manages access to device files. It includes both |
|
creation of new device files (using mknod), and access to the |
|
existing device files. |
|
|
|
Cgroup v2 device controller has no interface files and is implemented |
|
on top of cgroup BPF. To control access to device files, a user may |
|
create bpf programs of the BPF_CGROUP_DEVICE type and attach them |
|
to cgroups. On an attempt to access a device file, corresponding |
|
BPF programs will be executed, and depending on the return value |
|
the attempt will succeed or fail with -EPERM. |
|
|
|
A BPF_CGROUP_DEVICE program takes a pointer to the bpf_cgroup_dev_ctx |
|
structure, which describes the device access attempt: access type |
|
(mknod/read/write) and device (type, major and minor numbers). |
|
If the program returns 0, the attempt fails with -EPERM, otherwise |
|
it succeeds. |
|
|
|
An example of BPF_CGROUP_DEVICE program may be found in the kernel |
|
source tree in the tools/testing/selftests/bpf/progs/dev_cgroup.c file. |
|
|
|
|
|
RDMA |
|
---- |
|
|
|
The "rdma" controller regulates the distribution and accounting of |
|
RDMA resources. |
|
|
|
RDMA Interface Files |
|
~~~~~~~~~~~~~~~~~~~~ |
|
|
|
rdma.max |
|
A readwrite nested-keyed file that exists for all the cgroups |
|
except root that describes current configured resource limit |
|
for a RDMA/IB device. |
|
|
|
Lines are keyed by device name and are not ordered. |
|
Each line contains space separated resource name and its configured |
|
limit that can be distributed. |
|
|
|
The following nested keys are defined. |
|
|
|
========== ============================= |
|
hca_handle Maximum number of HCA Handles |
|
hca_object Maximum number of HCA Objects |
|
========== ============================= |
|
|
|
An example for mlx4 and ocrdma device follows:: |
|
|
|
mlx4_0 hca_handle=2 hca_object=2000 |
|
ocrdma1 hca_handle=3 hca_object=max |
|
|
|
rdma.current |
|
A read-only file that describes current resource usage. |
|
It exists for all the cgroup except root. |
|
|
|
An example for mlx4 and ocrdma device follows:: |
|
|
|
mlx4_0 hca_handle=1 hca_object=20 |
|
ocrdma1 hca_handle=1 hca_object=23 |
|
|
|
HugeTLB |
|
------- |
|
|
|
The HugeTLB controller allows to limit the HugeTLB usage per control group and |
|
enforces the controller limit during page fault. |
|
|
|
HugeTLB Interface Files |
|
~~~~~~~~~~~~~~~~~~~~~~~ |
|
|
|
hugetlb.<hugepagesize>.current |
|
Show current usage for "hugepagesize" hugetlb. It exists for all |
|
the cgroup except root. |
|
|
|
hugetlb.<hugepagesize>.max |
|
Set/show the hard limit of "hugepagesize" hugetlb usage. |
|
The default value is "max". It exists for all the cgroup except root. |
|
|
|
hugetlb.<hugepagesize>.events |
|
A read-only flat-keyed file which exists on non-root cgroups. |
|
|
|
max |
|
The number of allocation failure due to HugeTLB limit |
|
|
|
hugetlb.<hugepagesize>.events.local |
|
Similar to hugetlb.<hugepagesize>.events but the fields in the file |
|
are local to the cgroup i.e. not hierarchical. The file modified event |
|
generated on this file reflects only the local events. |
|
|
|
Misc |
|
---- |
|
|
|
The Miscellaneous cgroup provides the resource limiting and tracking |
|
mechanism for the scalar resources which cannot be abstracted like the other |
|
cgroup resources. Controller is enabled by the CONFIG_CGROUP_MISC config |
|
option. |
|
|
|
A resource can be added to the controller via enum misc_res_type{} in the |
|
include/linux/misc_cgroup.h file and the corresponding name via misc_res_name[] |
|
in the kernel/cgroup/misc.c file. Provider of the resource must set its |
|
capacity prior to using the resource by calling misc_cg_set_capacity(). |
|
|
|
Once a capacity is set then the resource usage can be updated using charge and |
|
uncharge APIs. All of the APIs to interact with misc controller are in |
|
include/linux/misc_cgroup.h. |
|
|
|
Misc Interface Files |
|
~~~~~~~~~~~~~~~~~~~~ |
|
|
|
Miscellaneous controller provides 3 interface files. If two misc resources (res_a and res_b) are registered then: |
|
|
|
misc.capacity |
|
A read-only flat-keyed file shown only in the root cgroup. It shows |
|
miscellaneous scalar resources available on the platform along with |
|
their quantities:: |
|
|
|
$ cat misc.capacity |
|
res_a 50 |
|
res_b 10 |
|
|
|
misc.current |
|
A read-only flat-keyed file shown in the non-root cgroups. It shows |
|
the current usage of the resources in the cgroup and its children.:: |
|
|
|
$ cat misc.current |
|
res_a 3 |
|
res_b 0 |
|
|
|
misc.max |
|
A read-write flat-keyed file shown in the non root cgroups. Allowed |
|
maximum usage of the resources in the cgroup and its children.:: |
|
|
|
$ cat misc.max |
|
res_a max |
|
res_b 4 |
|
|
|
Limit can be set by:: |
|
|
|
# echo res_a 1 > misc.max |
|
|
|
Limit can be set to max by:: |
|
|
|
# echo res_a max > misc.max |
|
|
|
Limits can be set higher than the capacity value in the misc.capacity |
|
file. |
|
|
|
Migration and Ownership |
|
~~~~~~~~~~~~~~~~~~~~~~~ |
|
|
|
A miscellaneous scalar resource is charged to the cgroup in which it is used |
|
first, and stays charged to that cgroup until that resource is freed. Migrating |
|
a process to a different cgroup does not move the charge to the destination |
|
cgroup where the process has moved. |
|
|
|
Others |
|
------ |
|
|
|
perf_event |
|
~~~~~~~~~~ |
|
|
|
perf_event controller, if not mounted on a legacy hierarchy, is |
|
automatically enabled on the v2 hierarchy so that perf events can |
|
always be filtered by cgroup v2 path. The controller can still be |
|
moved to a legacy hierarchy after v2 hierarchy is populated. |
|
|
|
|
|
Non-normative information |
|
------------------------- |
|
|
|
This section contains information that isn't considered to be a part of |
|
the stable kernel API and so is subject to change. |
|
|
|
|
|
CPU controller root cgroup process behaviour |
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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|
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When distributing CPU cycles in the root cgroup each thread in this |
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cgroup is treated as if it was hosted in a separate child cgroup of the |
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root cgroup. This child cgroup weight is dependent on its thread nice |
|
level. |
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|
|
For details of this mapping see sched_prio_to_weight array in |
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kernel/sched/core.c file (values from this array should be scaled |
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appropriately so the neutral - nice 0 - value is 100 instead of 1024). |
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IO controller root cgroup process behaviour |
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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Root cgroup processes are hosted in an implicit leaf child node. |
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When distributing IO resources this implicit child node is taken into |
|
account as if it was a normal child cgroup of the root cgroup with a |
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weight value of 200. |
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Namespace |
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========= |
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Basics |
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------ |
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|
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cgroup namespace provides a mechanism to virtualize the view of the |
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"/proc/$PID/cgroup" file and cgroup mounts. The CLONE_NEWCGROUP clone |
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flag can be used with clone(2) and unshare(2) to create a new cgroup |
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namespace. The process running inside the cgroup namespace will have |
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its "/proc/$PID/cgroup" output restricted to cgroupns root. The |
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cgroupns root is the cgroup of the process at the time of creation of |
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the cgroup namespace. |
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|
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Without cgroup namespace, the "/proc/$PID/cgroup" file shows the |
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complete path of the cgroup of a process. In a container setup where |
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a set of cgroups and namespaces are intended to isolate processes the |
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"/proc/$PID/cgroup" file may leak potential system level information |
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to the isolated processes. For example:: |
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|
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# cat /proc/self/cgroup |
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0::/batchjobs/container_id1 |
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The path '/batchjobs/container_id1' can be considered as system-data |
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and undesirable to expose to the isolated processes. cgroup namespace |
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can be used to restrict visibility of this path. For example, before |
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creating a cgroup namespace, one would see:: |
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# ls -l /proc/self/ns/cgroup |
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lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835] |
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# cat /proc/self/cgroup |
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0::/batchjobs/container_id1 |
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|
After unsharing a new namespace, the view changes:: |
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# ls -l /proc/self/ns/cgroup |
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lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183] |
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# cat /proc/self/cgroup |
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0::/ |
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When some thread from a multi-threaded process unshares its cgroup |
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namespace, the new cgroupns gets applied to the entire process (all |
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the threads). This is natural for the v2 hierarchy; however, for the |
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legacy hierarchies, this may be unexpected. |
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|
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A cgroup namespace is alive as long as there are processes inside or |
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mounts pinning it. When the last usage goes away, the cgroup |
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namespace is destroyed. The cgroupns root and the actual cgroups |
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remain. |
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The Root and Views |
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------------------ |
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The 'cgroupns root' for a cgroup namespace is the cgroup in which the |
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process calling unshare(2) is running. For example, if a process in |
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/batchjobs/container_id1 cgroup calls unshare, cgroup |
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/batchjobs/container_id1 becomes the cgroupns root. For the |
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init_cgroup_ns, this is the real root ('/') cgroup. |
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|
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The cgroupns root cgroup does not change even if the namespace creator |
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process later moves to a different cgroup:: |
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|
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# ~/unshare -c # unshare cgroupns in some cgroup |
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# cat /proc/self/cgroup |
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0::/ |
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# mkdir sub_cgrp_1 |
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# echo 0 > sub_cgrp_1/cgroup.procs |
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# cat /proc/self/cgroup |
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0::/sub_cgrp_1 |
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|
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Each process gets its namespace-specific view of "/proc/$PID/cgroup" |
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Processes running inside the cgroup namespace will be able to see |
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cgroup paths (in /proc/self/cgroup) only inside their root cgroup. |
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From within an unshared cgroupns:: |
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|
|
# sleep 100000 & |
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[1] 7353 |
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# echo 7353 > sub_cgrp_1/cgroup.procs |
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# cat /proc/7353/cgroup |
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0::/sub_cgrp_1 |
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|
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From the initial cgroup namespace, the real cgroup path will be |
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visible:: |
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$ cat /proc/7353/cgroup |
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0::/batchjobs/container_id1/sub_cgrp_1 |
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From a sibling cgroup namespace (that is, a namespace rooted at a |
|
different cgroup), the cgroup path relative to its own cgroup |
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namespace root will be shown. For instance, if PID 7353's cgroup |
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namespace root is at '/batchjobs/container_id2', then it will see:: |
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# cat /proc/7353/cgroup |
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0::/../container_id2/sub_cgrp_1 |
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Note that the relative path always starts with '/' to indicate that |
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its relative to the cgroup namespace root of the caller. |
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|
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Migration and setns(2) |
|
---------------------- |
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|
|
Processes inside a cgroup namespace can move into and out of the |
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namespace root if they have proper access to external cgroups. For |
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example, from inside a namespace with cgroupns root at |
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/batchjobs/container_id1, and assuming that the global hierarchy is |
|
still accessible inside cgroupns:: |
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|
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# cat /proc/7353/cgroup |
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0::/sub_cgrp_1 |
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# echo 7353 > batchjobs/container_id2/cgroup.procs |
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# cat /proc/7353/cgroup |
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0::/../container_id2 |
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|
|
Note that this kind of setup is not encouraged. A task inside cgroup |
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namespace should only be exposed to its own cgroupns hierarchy. |
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|
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setns(2) to another cgroup namespace is allowed when: |
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|
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(a) the process has CAP_SYS_ADMIN against its current user namespace |
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(b) the process has CAP_SYS_ADMIN against the target cgroup |
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namespace's userns |
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|
|
No implicit cgroup changes happen with attaching to another cgroup |
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namespace. It is expected that the someone moves the attaching |
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process under the target cgroup namespace root. |
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Interaction with Other Namespaces |
|
--------------------------------- |
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Namespace specific cgroup hierarchy can be mounted by a process |
|
running inside a non-init cgroup namespace:: |
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|
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# mount -t cgroup2 none $MOUNT_POINT |
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|
This will mount the unified cgroup hierarchy with cgroupns root as the |
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filesystem root. The process needs CAP_SYS_ADMIN against its user and |
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mount namespaces. |
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The virtualization of /proc/self/cgroup file combined with restricting |
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the view of cgroup hierarchy by namespace-private cgroupfs mount |
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provides a properly isolated cgroup view inside the container. |
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|
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Information on Kernel Programming |
|
================================= |
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|
|
This section contains kernel programming information in the areas |
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where interacting with cgroup is necessary. cgroup core and |
|
controllers are not covered. |
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Filesystem Support for Writeback |
|
-------------------------------- |
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|
|
A filesystem can support cgroup writeback by updating |
|
address_space_operations->writepage[s]() to annotate bio's using the |
|
following two functions. |
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|
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wbc_init_bio(@wbc, @bio) |
|
Should be called for each bio carrying writeback data and |
|
associates the bio with the inode's owner cgroup and the |
|
corresponding request queue. This must be called after |
|
a queue (device) has been associated with the bio and |
|
before submission. |
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wbc_account_cgroup_owner(@wbc, @page, @bytes) |
|
Should be called for each data segment being written out. |
|
While this function doesn't care exactly when it's called |
|
during the writeback session, it's the easiest and most |
|
natural to call it as data segments are added to a bio. |
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|
|
With writeback bio's annotated, cgroup support can be enabled per |
|
super_block by setting SB_I_CGROUPWB in ->s_iflags. This allows for |
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selective disabling of cgroup writeback support which is helpful when |
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certain filesystem features, e.g. journaled data mode, are |
|
incompatible. |
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|
|
wbc_init_bio() binds the specified bio to its cgroup. Depending on |
|
the configuration, the bio may be executed at a lower priority and if |
|
the writeback session is holding shared resources, e.g. a journal |
|
entry, may lead to priority inversion. There is no one easy solution |
|
for the problem. Filesystems can try to work around specific problem |
|
cases by skipping wbc_init_bio() and using bio_associate_blkg() |
|
directly. |
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|
|
Deprecated v1 Core Features |
|
=========================== |
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- Multiple hierarchies including named ones are not supported. |
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|
- All v1 mount options are not supported. |
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- The "tasks" file is removed and "cgroup.procs" is not sorted. |
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|
|
- "cgroup.clone_children" is removed. |
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|
|
- /proc/cgroups is meaningless for v2. Use "cgroup.controllers" file |
|
at the root instead. |
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|
Issues with v1 and Rationales for v2 |
|
==================================== |
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|
|
Multiple Hierarchies |
|
-------------------- |
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|
|
cgroup v1 allowed an arbitrary number of hierarchies and each |
|
hierarchy could host any number of controllers. While this seemed to |
|
provide a high level of flexibility, it wasn't useful in practice. |
|
|
|
For example, as there is only one instance of each controller, utility |
|
type controllers such as freezer which can be useful in all |
|
hierarchies could only be used in one. The issue is exacerbated by |
|
the fact that controllers couldn't be moved to another hierarchy once |
|
hierarchies were populated. Another issue was that all controllers |
|
bound to a hierarchy were forced to have exactly the same view of the |
|
hierarchy. It wasn't possible to vary the granularity depending on |
|
the specific controller. |
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|
|
In practice, these issues heavily limited which controllers could be |
|
put on the same hierarchy and most configurations resorted to putting |
|
each controller on its own hierarchy. Only closely related ones, such |
|
as the cpu and cpuacct controllers, made sense to be put on the same |
|
hierarchy. This often meant that userland ended up managing multiple |
|
similar hierarchies repeating the same steps on each hierarchy |
|
whenever a hierarchy management operation was necessary. |
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|
|
Furthermore, support for multiple hierarchies came at a steep cost. |
|
It greatly complicated cgroup core implementation but more importantly |
|
the support for multiple hierarchies restricted how cgroup could be |
|
used in general and what controllers was able to do. |
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|
|
There was no limit on how many hierarchies there might be, which meant |
|
that a thread's cgroup membership couldn't be described in finite |
|
length. The key might contain any number of entries and was unlimited |
|
in length, which made it highly awkward to manipulate and led to |
|
addition of controllers which existed only to identify membership, |
|
which in turn exacerbated the original problem of proliferating number |
|
of hierarchies. |
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|
Also, as a controller couldn't have any expectation regarding the |
|
topologies of hierarchies other controllers might be on, each |
|
controller had to assume that all other controllers were attached to |
|
completely orthogonal hierarchies. This made it impossible, or at |
|
least very cumbersome, for controllers to cooperate with each other. |
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|
|
In most use cases, putting controllers on hierarchies which are |
|
completely orthogonal to each other isn't necessary. What usually is |
|
called for is the ability to have differing levels of granularity |
|
depending on the specific controller. In other words, hierarchy may |
|
be collapsed from leaf towards root when viewed from specific |
|
controllers. For example, a given configuration might not care about |
|
how memory is distributed beyond a certain level while still wanting |
|
to control how CPU cycles are distributed. |
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|
Thread Granularity |
|
------------------ |
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|
|
cgroup v1 allowed threads of a process to belong to different cgroups. |
|
This didn't make sense for some controllers and those controllers |
|
ended up implementing different ways to ignore such situations but |
|
much more importantly it blurred the line between API exposed to |
|
individual applications and system management interface. |
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|
|
Generally, in-process knowledge is available only to the process |
|
itself; thus, unlike service-level organization of processes, |
|
categorizing threads of a process requires active participation from |
|
the application which owns the target process. |
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|
|
cgroup v1 had an ambiguously defined delegation model which got abused |
|
in combination with thread granularity. cgroups were delegated to |
|
individual applications so that they can create and manage their own |
|
sub-hierarchies and control resource distributions along them. This |
|
effectively raised cgroup to the status of a syscall-like API exposed |
|
to lay programs. |
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|
|
First of all, cgroup has a fundamentally inadequate interface to be |
|
exposed this way. For a process to access its own knobs, it has to |
|
extract the path on the target hierarchy from /proc/self/cgroup, |
|
construct the path by appending the name of the knob to the path, open |
|
and then read and/or write to it. This is not only extremely clunky |
|
and unusual but also inherently racy. There is no conventional way to |
|
define transaction across the required steps and nothing can guarantee |
|
that the process would actually be operating on its own sub-hierarchy. |
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|
|
cgroup controllers implemented a number of knobs which would never be |
|
accepted as public APIs because they were just adding control knobs to |
|
system-management pseudo filesystem. cgroup ended up with interface |
|
knobs which were not properly abstracted or refined and directly |
|
revealed kernel internal details. These knobs got exposed to |
|
individual applications through the ill-defined delegation mechanism |
|
effectively abusing cgroup as a shortcut to implementing public APIs |
|
without going through the required scrutiny. |
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|
|
This was painful for both userland and kernel. Userland ended up with |
|
misbehaving and poorly abstracted interfaces and kernel exposing and |
|
locked into constructs inadvertently. |
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|
|
|
|
Competition Between Inner Nodes and Threads |
|
------------------------------------------- |
|
|
|
cgroup v1 allowed threads to be in any cgroups which created an |
|
interesting problem where threads belonging to a parent cgroup and its |
|
children cgroups competed for resources. This was nasty as two |
|
different types of entities competed and there was no obvious way to |
|
settle it. Different controllers did different things. |
|
|
|
The cpu controller considered threads and cgroups as equivalents and |
|
mapped nice levels to cgroup weights. This worked for some cases but |
|
fell flat when children wanted to be allocated specific ratios of CPU |
|
cycles and the number of internal threads fluctuated - the ratios |
|
constantly changed as the number of competing entities fluctuated. |
|
There also were other issues. The mapping from nice level to weight |
|
wasn't obvious or universal, and there were various other knobs which |
|
simply weren't available for threads. |
|
|
|
The io controller implicitly created a hidden leaf node for each |
|
cgroup to host the threads. The hidden leaf had its own copies of all |
|
the knobs with ``leaf_`` prefixed. While this allowed equivalent |
|
control over internal threads, it was with serious drawbacks. It |
|
always added an extra layer of nesting which wouldn't be necessary |
|
otherwise, made the interface messy and significantly complicated the |
|
implementation. |
|
|
|
The memory controller didn't have a way to control what happened |
|
between internal tasks and child cgroups and the behavior was not |
|
clearly defined. There were attempts to add ad-hoc behaviors and |
|
knobs to tailor the behavior to specific workloads which would have |
|
led to problems extremely difficult to resolve in the long term. |
|
|
|
Multiple controllers struggled with internal tasks and came up with |
|
different ways to deal with it; unfortunately, all the approaches were |
|
severely flawed and, furthermore, the widely different behaviors |
|
made cgroup as a whole highly inconsistent. |
|
|
|
This clearly is a problem which needs to be addressed from cgroup core |
|
in a uniform way. |
|
|
|
|
|
Other Interface Issues |
|
---------------------- |
|
|
|
cgroup v1 grew without oversight and developed a large number of |
|
idiosyncrasies and inconsistencies. One issue on the cgroup core side |
|
was how an empty cgroup was notified - a userland helper binary was |
|
forked and executed for each event. The event delivery wasn't |
|
recursive or delegatable. The limitations of the mechanism also led |
|
to in-kernel event delivery filtering mechanism further complicating |
|
the interface. |
|
|
|
Controller interfaces were problematic too. An extreme example is |
|
controllers completely ignoring hierarchical organization and treating |
|
all cgroups as if they were all located directly under the root |
|
cgroup. Some controllers exposed a large amount of inconsistent |
|
implementation details to userland. |
|
|
|
There also was no consistency across controllers. When a new cgroup |
|
was created, some controllers defaulted to not imposing extra |
|
restrictions while others disallowed any resource usage until |
|
explicitly configured. Configuration knobs for the same type of |
|
control used widely differing naming schemes and formats. Statistics |
|
and information knobs were named arbitrarily and used different |
|
formats and units even in the same controller. |
|
|
|
cgroup v2 establishes common conventions where appropriate and updates |
|
controllers so that they expose minimal and consistent interfaces. |
|
|
|
|
|
Controller Issues and Remedies |
|
------------------------------ |
|
|
|
Memory |
|
~~~~~~ |
|
|
|
The original lower boundary, the soft limit, is defined as a limit |
|
that is per default unset. As a result, the set of cgroups that |
|
global reclaim prefers is opt-in, rather than opt-out. The costs for |
|
optimizing these mostly negative lookups are so high that the |
|
implementation, despite its enormous size, does not even provide the |
|
basic desirable behavior. First off, the soft limit has no |
|
hierarchical meaning. All configured groups are organized in a global |
|
rbtree and treated like equal peers, regardless where they are located |
|
in the hierarchy. This makes subtree delegation impossible. Second, |
|
the soft limit reclaim pass is so aggressive that it not just |
|
introduces high allocation latencies into the system, but also impacts |
|
system performance due to overreclaim, to the point where the feature |
|
becomes self-defeating. |
|
|
|
The memory.low boundary on the other hand is a top-down allocated |
|
reserve. A cgroup enjoys reclaim protection when it's within its |
|
effective low, which makes delegation of subtrees possible. It also |
|
enjoys having reclaim pressure proportional to its overage when |
|
above its effective low. |
|
|
|
The original high boundary, the hard limit, is defined as a strict |
|
limit that can not budge, even if the OOM killer has to be called. |
|
But this generally goes against the goal of making the most out of the |
|
available memory. The memory consumption of workloads varies during |
|
runtime, and that requires users to overcommit. But doing that with a |
|
strict upper limit requires either a fairly accurate prediction of the |
|
working set size or adding slack to the limit. Since working set size |
|
estimation is hard and error prone, and getting it wrong results in |
|
OOM kills, most users tend to err on the side of a looser limit and |
|
end up wasting precious resources. |
|
|
|
The memory.high boundary on the other hand can be set much more |
|
conservatively. When hit, it throttles allocations by forcing them |
|
into direct reclaim to work off the excess, but it never invokes the |
|
OOM killer. As a result, a high boundary that is chosen too |
|
aggressively will not terminate the processes, but instead it will |
|
lead to gradual performance degradation. The user can monitor this |
|
and make corrections until the minimal memory footprint that still |
|
gives acceptable performance is found. |
|
|
|
In extreme cases, with many concurrent allocations and a complete |
|
breakdown of reclaim progress within the group, the high boundary can |
|
be exceeded. But even then it's mostly better to satisfy the |
|
allocation from the slack available in other groups or the rest of the |
|
system than killing the group. Otherwise, memory.max is there to |
|
limit this type of spillover and ultimately contain buggy or even |
|
malicious applications. |
|
|
|
Setting the original memory.limit_in_bytes below the current usage was |
|
subject to a race condition, where concurrent charges could cause the |
|
limit setting to fail. memory.max on the other hand will first set the |
|
limit to prevent new charges, and then reclaim and OOM kill until the |
|
new limit is met - or the task writing to memory.max is killed. |
|
|
|
The combined memory+swap accounting and limiting is replaced by real |
|
control over swap space. |
|
|
|
The main argument for a combined memory+swap facility in the original |
|
cgroup design was that global or parental pressure would always be |
|
able to swap all anonymous memory of a child group, regardless of the |
|
child's own (possibly untrusted) configuration. However, untrusted |
|
groups can sabotage swapping by other means - such as referencing its |
|
anonymous memory in a tight loop - and an admin can not assume full |
|
swappability when overcommitting untrusted jobs. |
|
|
|
For trusted jobs, on the other hand, a combined counter is not an |
|
intuitive userspace interface, and it flies in the face of the idea |
|
that cgroup controllers should account and limit specific physical |
|
resources. Swap space is a resource like all others in the system, |
|
and that's why unified hierarchy allows distributing it separately.
|
|
|