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756 lines
29 KiB
756 lines
29 KiB
========================= |
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CPU hotplug in the Kernel |
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========================= |
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:Date: September, 2021 |
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:Author: Sebastian Andrzej Siewior <[email protected]>, |
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Rusty Russell <[email protected]>, |
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Srivatsa Vaddagiri <[email protected]>, |
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Ashok Raj <[email protected]>, |
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Joel Schopp <[email protected]>, |
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Thomas Gleixner <[email protected]> |
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Introduction |
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============ |
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Modern advances in system architectures have introduced advanced error |
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reporting and correction capabilities in processors. There are couple OEMS that |
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support NUMA hardware which are hot pluggable as well, where physical node |
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insertion and removal require support for CPU hotplug. |
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Such advances require CPUs available to a kernel to be removed either for |
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provisioning reasons, or for RAS purposes to keep an offending CPU off |
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system execution path. Hence the need for CPU hotplug support in the |
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Linux kernel. |
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A more novel use of CPU-hotplug support is its use today in suspend resume |
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support for SMP. Dual-core and HT support makes even a laptop run SMP kernels |
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which didn't support these methods. |
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Command Line Switches |
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===================== |
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``maxcpus=n`` |
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Restrict boot time CPUs to *n*. Say if you have four CPUs, using |
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``maxcpus=2`` will only boot two. You can choose to bring the |
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other CPUs later online. |
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``nr_cpus=n`` |
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Restrict the total amount of CPUs the kernel will support. If the number |
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supplied here is lower than the number of physically available CPUs, then |
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those CPUs can not be brought online later. |
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``additional_cpus=n`` |
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Use this to limit hotpluggable CPUs. This option sets |
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``cpu_possible_mask = cpu_present_mask + additional_cpus`` |
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This option is limited to the IA64 architecture. |
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``possible_cpus=n`` |
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This option sets ``possible_cpus`` bits in ``cpu_possible_mask``. |
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This option is limited to the X86 and S390 architecture. |
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``cpu0_hotplug`` |
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Allow to shutdown CPU0. |
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This option is limited to the X86 architecture. |
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CPU maps |
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======== |
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``cpu_possible_mask`` |
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Bitmap of possible CPUs that can ever be available in the |
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system. This is used to allocate some boot time memory for per_cpu variables |
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that aren't designed to grow/shrink as CPUs are made available or removed. |
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Once set during boot time discovery phase, the map is static, i.e no bits |
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are added or removed anytime. Trimming it accurately for your system needs |
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upfront can save some boot time memory. |
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``cpu_online_mask`` |
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Bitmap of all CPUs currently online. Its set in ``__cpu_up()`` |
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after a CPU is available for kernel scheduling and ready to receive |
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interrupts from devices. Its cleared when a CPU is brought down using |
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``__cpu_disable()``, before which all OS services including interrupts are |
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migrated to another target CPU. |
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``cpu_present_mask`` |
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Bitmap of CPUs currently present in the system. Not all |
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of them may be online. When physical hotplug is processed by the relevant |
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subsystem (e.g ACPI) can change and new bit either be added or removed |
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from the map depending on the event is hot-add/hot-remove. There are currently |
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no locking rules as of now. Typical usage is to init topology during boot, |
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at which time hotplug is disabled. |
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You really don't need to manipulate any of the system CPU maps. They should |
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be read-only for most use. When setting up per-cpu resources almost always use |
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``cpu_possible_mask`` or ``for_each_possible_cpu()`` to iterate. To macro |
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``for_each_cpu()`` can be used to iterate over a custom CPU mask. |
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Never use anything other than ``cpumask_t`` to represent bitmap of CPUs. |
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Using CPU hotplug |
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================= |
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The kernel option *CONFIG_HOTPLUG_CPU* needs to be enabled. It is currently |
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available on multiple architectures including ARM, MIPS, PowerPC and X86. The |
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configuration is done via the sysfs interface:: |
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$ ls -lh /sys/devices/system/cpu |
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total 0 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu0 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu1 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu2 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu3 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu4 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu5 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu6 |
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drwxr-xr-x 9 root root 0 Dec 21 16:33 cpu7 |
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drwxr-xr-x 2 root root 0 Dec 21 16:33 hotplug |
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-r--r--r-- 1 root root 4.0K Dec 21 16:33 offline |
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-r--r--r-- 1 root root 4.0K Dec 21 16:33 online |
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-r--r--r-- 1 root root 4.0K Dec 21 16:33 possible |
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-r--r--r-- 1 root root 4.0K Dec 21 16:33 present |
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The files *offline*, *online*, *possible*, *present* represent the CPU masks. |
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Each CPU folder contains an *online* file which controls the logical on (1) and |
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off (0) state. To logically shutdown CPU4:: |
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$ echo 0 > /sys/devices/system/cpu/cpu4/online |
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smpboot: CPU 4 is now offline |
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Once the CPU is shutdown, it will be removed from */proc/interrupts*, |
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*/proc/cpuinfo* and should also not be shown visible by the *top* command. To |
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bring CPU4 back online:: |
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$ echo 1 > /sys/devices/system/cpu/cpu4/online |
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smpboot: Booting Node 0 Processor 4 APIC 0x1 |
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The CPU is usable again. This should work on all CPUs. CPU0 is often special |
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and excluded from CPU hotplug. On X86 the kernel option |
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*CONFIG_BOOTPARAM_HOTPLUG_CPU0* has to be enabled in order to be able to |
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shutdown CPU0. Alternatively the kernel command option *cpu0_hotplug* can be |
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used. Some known dependencies of CPU0: |
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* Resume from hibernate/suspend. Hibernate/suspend will fail if CPU0 is offline. |
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* PIC interrupts. CPU0 can't be removed if a PIC interrupt is detected. |
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Please let Fenghua Yu <[email protected]> know if you find any dependencies |
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on CPU0. |
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The CPU hotplug coordination |
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============================ |
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The offline case |
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---------------- |
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Once a CPU has been logically shutdown the teardown callbacks of registered |
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hotplug states will be invoked, starting with ``CPUHP_ONLINE`` and terminating |
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at state ``CPUHP_OFFLINE``. This includes: |
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* If tasks are frozen due to a suspend operation then *cpuhp_tasks_frozen* |
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will be set to true. |
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* All processes are migrated away from this outgoing CPU to new CPUs. |
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The new CPU is chosen from each process' current cpuset, which may be |
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a subset of all online CPUs. |
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* All interrupts targeted to this CPU are migrated to a new CPU |
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* timers are also migrated to a new CPU |
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* Once all services are migrated, kernel calls an arch specific routine |
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``__cpu_disable()`` to perform arch specific cleanup. |
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The CPU hotplug API |
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=================== |
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CPU hotplug state machine |
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------------------------- |
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CPU hotplug uses a trivial state machine with a linear state space from |
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CPUHP_OFFLINE to CPUHP_ONLINE. Each state has a startup and a teardown |
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callback. |
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When a CPU is onlined, the startup callbacks are invoked sequentially until |
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the state CPUHP_ONLINE is reached. They can also be invoked when the |
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callbacks of a state are set up or an instance is added to a multi-instance |
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state. |
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When a CPU is offlined the teardown callbacks are invoked in the reverse |
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order sequentially until the state CPUHP_OFFLINE is reached. They can also |
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be invoked when the callbacks of a state are removed or an instance is |
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removed from a multi-instance state. |
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If a usage site requires only a callback in one direction of the hotplug |
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operations (CPU online or CPU offline) then the other not-required callback |
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can be set to NULL when the state is set up. |
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The state space is divided into three sections: |
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* The PREPARE section |
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The PREPARE section covers the state space from CPUHP_OFFLINE to |
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CPUHP_BRINGUP_CPU. |
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The startup callbacks in this section are invoked before the CPU is |
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started during a CPU online operation. The teardown callbacks are invoked |
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after the CPU has become dysfunctional during a CPU offline operation. |
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The callbacks are invoked on a control CPU as they can't obviously run on |
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the hotplugged CPU which is either not yet started or has become |
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dysfunctional already. |
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The startup callbacks are used to setup resources which are required to |
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bring a CPU successfully online. The teardown callbacks are used to free |
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resources or to move pending work to an online CPU after the hotplugged |
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CPU became dysfunctional. |
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The startup callbacks are allowed to fail. If a callback fails, the CPU |
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online operation is aborted and the CPU is brought down to the previous |
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state (usually CPUHP_OFFLINE) again. |
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The teardown callbacks in this section are not allowed to fail. |
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* The STARTING section |
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The STARTING section covers the state space between CPUHP_BRINGUP_CPU + 1 |
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and CPUHP_AP_ONLINE. |
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The startup callbacks in this section are invoked on the hotplugged CPU |
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with interrupts disabled during a CPU online operation in the early CPU |
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setup code. The teardown callbacks are invoked with interrupts disabled |
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on the hotplugged CPU during a CPU offline operation shortly before the |
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CPU is completely shut down. |
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The callbacks in this section are not allowed to fail. |
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The callbacks are used for low level hardware initialization/shutdown and |
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for core subsystems. |
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* The ONLINE section |
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The ONLINE section covers the state space between CPUHP_AP_ONLINE + 1 and |
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CPUHP_ONLINE. |
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The startup callbacks in this section are invoked on the hotplugged CPU |
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during a CPU online operation. The teardown callbacks are invoked on the |
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hotplugged CPU during a CPU offline operation. |
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The callbacks are invoked in the context of the per CPU hotplug thread, |
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which is pinned on the hotplugged CPU. The callbacks are invoked with |
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interrupts and preemption enabled. |
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The callbacks are allowed to fail. When a callback fails the hotplug |
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operation is aborted and the CPU is brought back to the previous state. |
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CPU online/offline operations |
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----------------------------- |
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A successful online operation looks like this:: |
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[CPUHP_OFFLINE] |
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[CPUHP_OFFLINE + 1]->startup() -> success |
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[CPUHP_OFFLINE + 2]->startup() -> success |
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[CPUHP_OFFLINE + 3] -> skipped because startup == NULL |
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... |
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[CPUHP_BRINGUP_CPU]->startup() -> success |
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=== End of PREPARE section |
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[CPUHP_BRINGUP_CPU + 1]->startup() -> success |
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... |
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[CPUHP_AP_ONLINE]->startup() -> success |
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=== End of STARTUP section |
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[CPUHP_AP_ONLINE + 1]->startup() -> success |
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... |
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[CPUHP_ONLINE - 1]->startup() -> success |
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[CPUHP_ONLINE] |
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A successful offline operation looks like this:: |
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[CPUHP_ONLINE] |
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[CPUHP_ONLINE - 1]->teardown() -> success |
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... |
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[CPUHP_AP_ONLINE + 1]->teardown() -> success |
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=== Start of STARTUP section |
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[CPUHP_AP_ONLINE]->teardown() -> success |
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... |
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[CPUHP_BRINGUP_ONLINE - 1]->teardown() |
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... |
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=== Start of PREPARE section |
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[CPUHP_BRINGUP_CPU]->teardown() |
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[CPUHP_OFFLINE + 3]->teardown() |
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[CPUHP_OFFLINE + 2] -> skipped because teardown == NULL |
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[CPUHP_OFFLINE + 1]->teardown() |
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[CPUHP_OFFLINE] |
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A failed online operation looks like this:: |
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[CPUHP_OFFLINE] |
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[CPUHP_OFFLINE + 1]->startup() -> success |
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[CPUHP_OFFLINE + 2]->startup() -> success |
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[CPUHP_OFFLINE + 3] -> skipped because startup == NULL |
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... |
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[CPUHP_BRINGUP_CPU]->startup() -> success |
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=== End of PREPARE section |
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[CPUHP_BRINGUP_CPU + 1]->startup() -> success |
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... |
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[CPUHP_AP_ONLINE]->startup() -> success |
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=== End of STARTUP section |
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[CPUHP_AP_ONLINE + 1]->startup() -> success |
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--- |
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[CPUHP_AP_ONLINE + N]->startup() -> fail |
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[CPUHP_AP_ONLINE + (N - 1)]->teardown() |
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... |
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[CPUHP_AP_ONLINE + 1]->teardown() |
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=== Start of STARTUP section |
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[CPUHP_AP_ONLINE]->teardown() |
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... |
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[CPUHP_BRINGUP_ONLINE - 1]->teardown() |
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... |
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=== Start of PREPARE section |
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[CPUHP_BRINGUP_CPU]->teardown() |
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[CPUHP_OFFLINE + 3]->teardown() |
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[CPUHP_OFFLINE + 2] -> skipped because teardown == NULL |
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[CPUHP_OFFLINE + 1]->teardown() |
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[CPUHP_OFFLINE] |
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A failed offline operation looks like this:: |
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[CPUHP_ONLINE] |
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[CPUHP_ONLINE - 1]->teardown() -> success |
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... |
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[CPUHP_ONLINE - N]->teardown() -> fail |
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[CPUHP_ONLINE - (N - 1)]->startup() |
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... |
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[CPUHP_ONLINE - 1]->startup() |
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[CPUHP_ONLINE] |
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Recursive failures cannot be handled sensibly. Look at the following |
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example of a recursive fail due to a failed offline operation: :: |
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[CPUHP_ONLINE] |
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[CPUHP_ONLINE - 1]->teardown() -> success |
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... |
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[CPUHP_ONLINE - N]->teardown() -> fail |
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[CPUHP_ONLINE - (N - 1)]->startup() -> success |
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[CPUHP_ONLINE - (N - 2)]->startup() -> fail |
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The CPU hotplug state machine stops right here and does not try to go back |
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down again because that would likely result in an endless loop:: |
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[CPUHP_ONLINE - (N - 1)]->teardown() -> success |
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[CPUHP_ONLINE - N]->teardown() -> fail |
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[CPUHP_ONLINE - (N - 1)]->startup() -> success |
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[CPUHP_ONLINE - (N - 2)]->startup() -> fail |
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[CPUHP_ONLINE - (N - 1)]->teardown() -> success |
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[CPUHP_ONLINE - N]->teardown() -> fail |
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Lather, rinse and repeat. In this case the CPU left in state:: |
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[CPUHP_ONLINE - (N - 1)] |
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which at least lets the system make progress and gives the user a chance to |
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debug or even resolve the situation. |
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Allocating a state |
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------------------ |
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There are two ways to allocate a CPU hotplug state: |
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* Static allocation |
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Static allocation has to be used when the subsystem or driver has |
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ordering requirements versus other CPU hotplug states. E.g. the PERF core |
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startup callback has to be invoked before the PERF driver startup |
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callbacks during a CPU online operation. During a CPU offline operation |
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the driver teardown callbacks have to be invoked before the core teardown |
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callback. The statically allocated states are described by constants in |
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the cpuhp_state enum which can be found in include/linux/cpuhotplug.h. |
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Insert the state into the enum at the proper place so the ordering |
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requirements are fulfilled. The state constant has to be used for state |
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setup and removal. |
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Static allocation is also required when the state callbacks are not set |
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up at runtime and are part of the initializer of the CPU hotplug state |
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array in kernel/cpu.c. |
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* Dynamic allocation |
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When there are no ordering requirements for the state callbacks then |
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dynamic allocation is the preferred method. The state number is allocated |
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by the setup function and returned to the caller on success. |
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Only the PREPARE and ONLINE sections provide a dynamic allocation |
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range. The STARTING section does not as most of the callbacks in that |
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section have explicit ordering requirements. |
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Setup of a CPU hotplug state |
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---------------------------- |
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The core code provides the following functions to setup a state: |
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* cpuhp_setup_state(state, name, startup, teardown) |
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* cpuhp_setup_state_nocalls(state, name, startup, teardown) |
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* cpuhp_setup_state_cpuslocked(state, name, startup, teardown) |
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* cpuhp_setup_state_nocalls_cpuslocked(state, name, startup, teardown) |
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For cases where a driver or a subsystem has multiple instances and the same |
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CPU hotplug state callbacks need to be invoked for each instance, the CPU |
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hotplug core provides multi-instance support. The advantage over driver |
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specific instance lists is that the instance related functions are fully |
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serialized against CPU hotplug operations and provide the automatic |
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invocations of the state callbacks on add and removal. To set up such a |
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multi-instance state the following function is available: |
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* cpuhp_setup_state_multi(state, name, startup, teardown) |
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The @state argument is either a statically allocated state or one of the |
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constants for dynamically allocated states - CPUHP_PREPARE_DYN, |
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CPUHP_ONLINE_DYN - depending on the state section (PREPARE, ONLINE) for |
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which a dynamic state should be allocated. |
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The @name argument is used for sysfs output and for instrumentation. The |
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naming convention is "subsys:mode" or "subsys/driver:mode", |
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e.g. "perf:mode" or "perf/x86:mode". The common mode names are: |
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======== ======================================================= |
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prepare For states in the PREPARE section |
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dead For states in the PREPARE section which do not provide |
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a startup callback |
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starting For states in the STARTING section |
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dying For states in the STARTING section which do not provide |
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a startup callback |
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online For states in the ONLINE section |
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offline For states in the ONLINE section which do not provide |
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a startup callback |
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======== ======================================================= |
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As the @name argument is only used for sysfs and instrumentation other mode |
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descriptors can be used as well if they describe the nature of the state |
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better than the common ones. |
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Examples for @name arguments: "perf/online", "perf/x86:prepare", |
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"RCU/tree:dying", "sched/waitempty" |
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The @startup argument is a function pointer to the callback which should be |
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invoked during a CPU online operation. If the usage site does not require a |
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startup callback set the pointer to NULL. |
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The @teardown argument is a function pointer to the callback which should |
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be invoked during a CPU offline operation. If the usage site does not |
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require a teardown callback set the pointer to NULL. |
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The functions differ in the way how the installed callbacks are treated: |
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* cpuhp_setup_state_nocalls(), cpuhp_setup_state_nocalls_cpuslocked() |
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and cpuhp_setup_state_multi() only install the callbacks |
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* cpuhp_setup_state() and cpuhp_setup_state_cpuslocked() install the |
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callbacks and invoke the @startup callback (if not NULL) for all online |
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CPUs which have currently a state greater than the newly installed |
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state. Depending on the state section the callback is either invoked on |
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the current CPU (PREPARE section) or on each online CPU (ONLINE |
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section) in the context of the CPU's hotplug thread. |
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If a callback fails for CPU N then the teardown callback for CPU |
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0 .. N-1 is invoked to rollback the operation. The state setup fails, |
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the callbacks for the state are not installed and in case of dynamic |
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allocation the allocated state is freed. |
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|
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The state setup and the callback invocations are serialized against CPU |
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hotplug operations. If the setup function has to be called from a CPU |
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hotplug read locked region, then the _cpuslocked() variants have to be |
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used. These functions cannot be used from within CPU hotplug callbacks. |
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|
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The function return values: |
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======== =================================================================== |
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0 Statically allocated state was successfully set up |
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>0 Dynamically allocated state was successfully set up. |
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The returned number is the state number which was allocated. If |
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the state callbacks have to be removed later, e.g. module |
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removal, then this number has to be saved by the caller and used |
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as @state argument for the state remove function. For |
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multi-instance states the dynamically allocated state number is |
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also required as @state argument for the instance add/remove |
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operations. |
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|
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<0 Operation failed |
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======== =================================================================== |
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|
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Removal of a CPU hotplug state |
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------------------------------ |
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|
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To remove a previously set up state, the following functions are provided: |
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* cpuhp_remove_state(state) |
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* cpuhp_remove_state_nocalls(state) |
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* cpuhp_remove_state_nocalls_cpuslocked(state) |
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* cpuhp_remove_multi_state(state) |
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The @state argument is either a statically allocated state or the state |
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number which was allocated in the dynamic range by cpuhp_setup_state*(). If |
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the state is in the dynamic range, then the state number is freed and |
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available for dynamic allocation again. |
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|
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The functions differ in the way how the installed callbacks are treated: |
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|
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* cpuhp_remove_state_nocalls(), cpuhp_remove_state_nocalls_cpuslocked() |
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and cpuhp_remove_multi_state() only remove the callbacks. |
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* cpuhp_remove_state() removes the callbacks and invokes the teardown |
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callback (if not NULL) for all online CPUs which have currently a state |
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greater than the removed state. Depending on the state section the |
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callback is either invoked on the current CPU (PREPARE section) or on |
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each online CPU (ONLINE section) in the context of the CPU's hotplug |
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thread. |
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In order to complete the removal, the teardown callback should not fail. |
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|
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The state removal and the callback invocations are serialized against CPU |
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hotplug operations. If the remove function has to be called from a CPU |
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hotplug read locked region, then the _cpuslocked() variants have to be |
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used. These functions cannot be used from within CPU hotplug callbacks. |
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|
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If a multi-instance state is removed then the caller has to remove all |
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instances first. |
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Multi-Instance state instance management |
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---------------------------------------- |
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Once the multi-instance state is set up, instances can be added to the |
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state: |
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* cpuhp_state_add_instance(state, node) |
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* cpuhp_state_add_instance_nocalls(state, node) |
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|
|
The @state argument is either a statically allocated state or the state |
|
number which was allocated in the dynamic range by cpuhp_setup_state_multi(). |
|
|
|
The @node argument is a pointer to an hlist_node which is embedded in the |
|
instance's data structure. The pointer is handed to the multi-instance |
|
state callbacks and can be used by the callback to retrieve the instance |
|
via container_of(). |
|
|
|
The functions differ in the way how the installed callbacks are treated: |
|
|
|
* cpuhp_state_add_instance_nocalls() and only adds the instance to the |
|
multi-instance state's node list. |
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|
|
* cpuhp_state_add_instance() adds the instance and invokes the startup |
|
callback (if not NULL) associated with @state for all online CPUs which |
|
have currently a state greater than @state. The callback is only |
|
invoked for the to be added instance. Depending on the state section |
|
the callback is either invoked on the current CPU (PREPARE section) or |
|
on each online CPU (ONLINE section) in the context of the CPU's hotplug |
|
thread. |
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|
|
If a callback fails for CPU N then the teardown callback for CPU |
|
0 .. N-1 is invoked to rollback the operation, the function fails and |
|
the instance is not added to the node list of the multi-instance state. |
|
|
|
To remove an instance from the state's node list these functions are |
|
available: |
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|
|
* cpuhp_state_remove_instance(state, node) |
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* cpuhp_state_remove_instance_nocalls(state, node) |
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|
|
The arguments are the same as for the the cpuhp_state_add_instance*() |
|
variants above. |
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|
|
The functions differ in the way how the installed callbacks are treated: |
|
|
|
* cpuhp_state_remove_instance_nocalls() only removes the instance from the |
|
state's node list. |
|
|
|
* cpuhp_state_remove_instance() removes the instance and invokes the |
|
teardown callback (if not NULL) associated with @state for all online |
|
CPUs which have currently a state greater than @state. The callback is |
|
only invoked for the to be removed instance. Depending on the state |
|
section the callback is either invoked on the current CPU (PREPARE |
|
section) or on each online CPU (ONLINE section) in the context of the |
|
CPU's hotplug thread. |
|
|
|
In order to complete the removal, the teardown callback should not fail. |
|
|
|
The node list add/remove operations and the callback invocations are |
|
serialized against CPU hotplug operations. These functions cannot be used |
|
from within CPU hotplug callbacks and CPU hotplug read locked regions. |
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|
|
Examples |
|
-------- |
|
|
|
Setup and teardown a statically allocated state in the STARTING section for |
|
notifications on online and offline operations:: |
|
|
|
ret = cpuhp_setup_state(CPUHP_SUBSYS_STARTING, "subsys:starting", subsys_cpu_starting, subsys_cpu_dying); |
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if (ret < 0) |
|
return ret; |
|
.... |
|
cpuhp_remove_state(CPUHP_SUBSYS_STARTING); |
|
|
|
Setup and teardown a dynamically allocated state in the ONLINE section |
|
for notifications on offline operations:: |
|
|
|
state = cpuhp_setup_state(CPUHP_ONLINE_DYN, "subsys:offline", NULL, subsys_cpu_offline); |
|
if (state < 0) |
|
return state; |
|
.... |
|
cpuhp_remove_state(state); |
|
|
|
Setup and teardown a dynamically allocated state in the ONLINE section |
|
for notifications on online operations without invoking the callbacks:: |
|
|
|
state = cpuhp_setup_state_nocalls(CPUHP_ONLINE_DYN, "subsys:online", subsys_cpu_online, NULL); |
|
if (state < 0) |
|
return state; |
|
.... |
|
cpuhp_remove_state_nocalls(state); |
|
|
|
Setup, use and teardown a dynamically allocated multi-instance state in the |
|
ONLINE section for notifications on online and offline operation:: |
|
|
|
state = cpuhp_setup_state_multi(CPUHP_ONLINE_DYN, "subsys:online", subsys_cpu_online, subsys_cpu_offline); |
|
if (state < 0) |
|
return state; |
|
.... |
|
ret = cpuhp_state_add_instance(state, &inst1->node); |
|
if (ret) |
|
return ret; |
|
.... |
|
ret = cpuhp_state_add_instance(state, &inst2->node); |
|
if (ret) |
|
return ret; |
|
.... |
|
cpuhp_remove_instance(state, &inst1->node); |
|
.... |
|
cpuhp_remove_instance(state, &inst2->node); |
|
.... |
|
remove_multi_state(state); |
|
|
|
|
|
Testing of hotplug states |
|
========================= |
|
|
|
One way to verify whether a custom state is working as expected or not is to |
|
shutdown a CPU and then put it online again. It is also possible to put the CPU |
|
to certain state (for instance *CPUHP_AP_ONLINE*) and then go back to |
|
*CPUHP_ONLINE*. This would simulate an error one state after *CPUHP_AP_ONLINE* |
|
which would lead to rollback to the online state. |
|
|
|
All registered states are enumerated in ``/sys/devices/system/cpu/hotplug/states`` :: |
|
|
|
$ tail /sys/devices/system/cpu/hotplug/states |
|
138: mm/vmscan:online |
|
139: mm/vmstat:online |
|
140: lib/percpu_cnt:online |
|
141: acpi/cpu-drv:online |
|
142: base/cacheinfo:online |
|
143: virtio/net:online |
|
144: x86/mce:online |
|
145: printk:online |
|
168: sched:active |
|
169: online |
|
|
|
To rollback CPU4 to ``lib/percpu_cnt:online`` and back online just issue:: |
|
|
|
$ cat /sys/devices/system/cpu/cpu4/hotplug/state |
|
169 |
|
$ echo 140 > /sys/devices/system/cpu/cpu4/hotplug/target |
|
$ cat /sys/devices/system/cpu/cpu4/hotplug/state |
|
140 |
|
|
|
It is important to note that the teardown callback of state 140 have been |
|
invoked. And now get back online:: |
|
|
|
$ echo 169 > /sys/devices/system/cpu/cpu4/hotplug/target |
|
$ cat /sys/devices/system/cpu/cpu4/hotplug/state |
|
169 |
|
|
|
With trace events enabled, the individual steps are visible, too:: |
|
|
|
# TASK-PID CPU# TIMESTAMP FUNCTION |
|
# | | | | | |
|
bash-394 [001] 22.976: cpuhp_enter: cpu: 0004 target: 140 step: 169 (cpuhp_kick_ap_work) |
|
cpuhp/4-31 [004] 22.977: cpuhp_enter: cpu: 0004 target: 140 step: 168 (sched_cpu_deactivate) |
|
cpuhp/4-31 [004] 22.990: cpuhp_exit: cpu: 0004 state: 168 step: 168 ret: 0 |
|
cpuhp/4-31 [004] 22.991: cpuhp_enter: cpu: 0004 target: 140 step: 144 (mce_cpu_pre_down) |
|
cpuhp/4-31 [004] 22.992: cpuhp_exit: cpu: 0004 state: 144 step: 144 ret: 0 |
|
cpuhp/4-31 [004] 22.993: cpuhp_multi_enter: cpu: 0004 target: 140 step: 143 (virtnet_cpu_down_prep) |
|
cpuhp/4-31 [004] 22.994: cpuhp_exit: cpu: 0004 state: 143 step: 143 ret: 0 |
|
cpuhp/4-31 [004] 22.995: cpuhp_enter: cpu: 0004 target: 140 step: 142 (cacheinfo_cpu_pre_down) |
|
cpuhp/4-31 [004] 22.996: cpuhp_exit: cpu: 0004 state: 142 step: 142 ret: 0 |
|
bash-394 [001] 22.997: cpuhp_exit: cpu: 0004 state: 140 step: 169 ret: 0 |
|
bash-394 [005] 95.540: cpuhp_enter: cpu: 0004 target: 169 step: 140 (cpuhp_kick_ap_work) |
|
cpuhp/4-31 [004] 95.541: cpuhp_enter: cpu: 0004 target: 169 step: 141 (acpi_soft_cpu_online) |
|
cpuhp/4-31 [004] 95.542: cpuhp_exit: cpu: 0004 state: 141 step: 141 ret: 0 |
|
cpuhp/4-31 [004] 95.543: cpuhp_enter: cpu: 0004 target: 169 step: 142 (cacheinfo_cpu_online) |
|
cpuhp/4-31 [004] 95.544: cpuhp_exit: cpu: 0004 state: 142 step: 142 ret: 0 |
|
cpuhp/4-31 [004] 95.545: cpuhp_multi_enter: cpu: 0004 target: 169 step: 143 (virtnet_cpu_online) |
|
cpuhp/4-31 [004] 95.546: cpuhp_exit: cpu: 0004 state: 143 step: 143 ret: 0 |
|
cpuhp/4-31 [004] 95.547: cpuhp_enter: cpu: 0004 target: 169 step: 144 (mce_cpu_online) |
|
cpuhp/4-31 [004] 95.548: cpuhp_exit: cpu: 0004 state: 144 step: 144 ret: 0 |
|
cpuhp/4-31 [004] 95.549: cpuhp_enter: cpu: 0004 target: 169 step: 145 (console_cpu_notify) |
|
cpuhp/4-31 [004] 95.550: cpuhp_exit: cpu: 0004 state: 145 step: 145 ret: 0 |
|
cpuhp/4-31 [004] 95.551: cpuhp_enter: cpu: 0004 target: 169 step: 168 (sched_cpu_activate) |
|
cpuhp/4-31 [004] 95.552: cpuhp_exit: cpu: 0004 state: 168 step: 168 ret: 0 |
|
bash-394 [005] 95.553: cpuhp_exit: cpu: 0004 state: 169 step: 140 ret: 0 |
|
|
|
As it an be seen, CPU4 went down until timestamp 22.996 and then back up until |
|
95.552. All invoked callbacks including their return codes are visible in the |
|
trace. |
|
|
|
Architecture's requirements |
|
=========================== |
|
|
|
The following functions and configurations are required: |
|
|
|
``CONFIG_HOTPLUG_CPU`` |
|
This entry needs to be enabled in Kconfig |
|
|
|
``__cpu_up()`` |
|
Arch interface to bring up a CPU |
|
|
|
``__cpu_disable()`` |
|
Arch interface to shutdown a CPU, no more interrupts can be handled by the |
|
kernel after the routine returns. This includes the shutdown of the timer. |
|
|
|
``__cpu_die()`` |
|
This actually supposed to ensure death of the CPU. Actually look at some |
|
example code in other arch that implement CPU hotplug. The processor is taken |
|
down from the ``idle()`` loop for that specific architecture. ``__cpu_die()`` |
|
typically waits for some per_cpu state to be set, to ensure the processor dead |
|
routine is called to be sure positively. |
|
|
|
User Space Notification |
|
======================= |
|
|
|
After CPU successfully onlined or offline udev events are sent. A udev rule like:: |
|
|
|
SUBSYSTEM=="cpu", DRIVERS=="processor", DEVPATH=="/devices/system/cpu/*", RUN+="the_hotplug_receiver.sh" |
|
|
|
will receive all events. A script like:: |
|
|
|
#!/bin/sh |
|
|
|
if [ "${ACTION}" = "offline" ] |
|
then |
|
echo "CPU ${DEVPATH##*/} offline" |
|
|
|
elif [ "${ACTION}" = "online" ] |
|
then |
|
echo "CPU ${DEVPATH##*/} online" |
|
|
|
fi |
|
|
|
can process the event further. |
|
|
|
Kernel Inline Documentations Reference |
|
====================================== |
|
|
|
.. kernel-doc:: include/linux/cpuhotplug.h
|
|
|