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234 lines
7.2 KiB
234 lines
7.2 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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============ |
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x86 Topology |
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============ |
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This documents and clarifies the main aspects of x86 topology modelling and |
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representation in the kernel. Update/change when doing changes to the |
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respective code. |
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The architecture-agnostic topology definitions are in |
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Documentation/admin-guide/cputopology.rst. This file holds x86-specific |
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differences/specialities which must not necessarily apply to the generic |
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definitions. Thus, the way to read up on Linux topology on x86 is to start |
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with the generic one and look at this one in parallel for the x86 specifics. |
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Needless to say, code should use the generic functions - this file is *only* |
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here to *document* the inner workings of x86 topology. |
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Started by Thomas Gleixner <[email protected]> and Borislav Petkov <[email protected]>. |
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The main aim of the topology facilities is to present adequate interfaces to |
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code which needs to know/query/use the structure of the running system wrt |
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threads, cores, packages, etc. |
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The kernel does not care about the concept of physical sockets because a |
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socket has no relevance to software. It's an electromechanical component. In |
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the past a socket always contained a single package (see below), but with the |
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advent of Multi Chip Modules (MCM) a socket can hold more than one package. So |
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there might be still references to sockets in the code, but they are of |
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historical nature and should be cleaned up. |
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The topology of a system is described in the units of: |
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- packages |
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- cores |
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- threads |
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Package |
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======= |
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Packages contain a number of cores plus shared resources, e.g. DRAM |
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controller, shared caches etc. |
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Modern systems may also use the term 'Die' for package. |
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AMD nomenclature for package is 'Node'. |
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Package-related topology information in the kernel: |
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- cpuinfo_x86.x86_max_cores: |
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The number of cores in a package. This information is retrieved via CPUID. |
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- cpuinfo_x86.x86_max_dies: |
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The number of dies in a package. This information is retrieved via CPUID. |
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- cpuinfo_x86.cpu_die_id: |
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The physical ID of the die. This information is retrieved via CPUID. |
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- cpuinfo_x86.phys_proc_id: |
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The physical ID of the package. This information is retrieved via CPUID |
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and deduced from the APIC IDs of the cores in the package. |
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Modern systems use this value for the socket. There may be multiple |
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packages within a socket. This value may differ from cpu_die_id. |
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- cpuinfo_x86.logical_proc_id: |
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The logical ID of the package. As we do not trust BIOSes to enumerate the |
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packages in a consistent way, we introduced the concept of logical package |
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ID so we can sanely calculate the number of maximum possible packages in |
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the system and have the packages enumerated linearly. |
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- topology_max_packages(): |
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The maximum possible number of packages in the system. Helpful for per |
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package facilities to preallocate per package information. |
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- cpu_llc_id: |
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A per-CPU variable containing: |
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- On Intel, the first APIC ID of the list of CPUs sharing the Last Level |
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Cache |
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- On AMD, the Node ID or Core Complex ID containing the Last Level |
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Cache. In general, it is a number identifying an LLC uniquely on the |
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system. |
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Cores |
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===== |
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A core consists of 1 or more threads. It does not matter whether the threads |
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are SMT- or CMT-type threads. |
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AMDs nomenclature for a CMT core is "Compute Unit". The kernel always uses |
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"core". |
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Core-related topology information in the kernel: |
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- smp_num_siblings: |
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The number of threads in a core. The number of threads in a package can be |
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calculated by:: |
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threads_per_package = cpuinfo_x86.x86_max_cores * smp_num_siblings |
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Threads |
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======= |
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A thread is a single scheduling unit. It's the equivalent to a logical Linux |
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CPU. |
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AMDs nomenclature for CMT threads is "Compute Unit Core". The kernel always |
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uses "thread". |
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Thread-related topology information in the kernel: |
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- topology_core_cpumask(): |
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The cpumask contains all online threads in the package to which a thread |
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belongs. |
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The number of online threads is also printed in /proc/cpuinfo "siblings." |
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- topology_sibling_cpumask(): |
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The cpumask contains all online threads in the core to which a thread |
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belongs. |
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- topology_logical_package_id(): |
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The logical package ID to which a thread belongs. |
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- topology_physical_package_id(): |
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The physical package ID to which a thread belongs. |
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- topology_core_id(); |
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The ID of the core to which a thread belongs. It is also printed in /proc/cpuinfo |
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"core_id." |
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System topology examples |
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======================== |
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.. note:: |
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The alternative Linux CPU enumeration depends on how the BIOS enumerates the |
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threads. Many BIOSes enumerate all threads 0 first and then all threads 1. |
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That has the "advantage" that the logical Linux CPU numbers of threads 0 stay |
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the same whether threads are enabled or not. That's merely an implementation |
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detail and has no practical impact. |
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1) Single Package, Single Core:: |
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[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 |
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2) Single Package, Dual Core |
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a) One thread per core:: |
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[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 |
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-> [core 1] -> [thread 0] -> Linux CPU 1 |
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b) Two threads per core:: |
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[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 |
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-> [thread 1] -> Linux CPU 1 |
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-> [core 1] -> [thread 0] -> Linux CPU 2 |
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-> [thread 1] -> Linux CPU 3 |
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Alternative enumeration:: |
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[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 |
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-> [thread 1] -> Linux CPU 2 |
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-> [core 1] -> [thread 0] -> Linux CPU 1 |
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-> [thread 1] -> Linux CPU 3 |
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AMD nomenclature for CMT systems:: |
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[node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0 |
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-> [Compute Unit Core 1] -> Linux CPU 1 |
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-> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2 |
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-> [Compute Unit Core 1] -> Linux CPU 3 |
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4) Dual Package, Dual Core |
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a) One thread per core:: |
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[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 |
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-> [core 1] -> [thread 0] -> Linux CPU 1 |
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[package 1] -> [core 0] -> [thread 0] -> Linux CPU 2 |
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-> [core 1] -> [thread 0] -> Linux CPU 3 |
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b) Two threads per core:: |
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[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 |
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-> [thread 1] -> Linux CPU 1 |
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-> [core 1] -> [thread 0] -> Linux CPU 2 |
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-> [thread 1] -> Linux CPU 3 |
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[package 1] -> [core 0] -> [thread 0] -> Linux CPU 4 |
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-> [thread 1] -> Linux CPU 5 |
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-> [core 1] -> [thread 0] -> Linux CPU 6 |
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-> [thread 1] -> Linux CPU 7 |
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Alternative enumeration:: |
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[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 |
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-> [thread 1] -> Linux CPU 4 |
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-> [core 1] -> [thread 0] -> Linux CPU 1 |
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-> [thread 1] -> Linux CPU 5 |
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[package 1] -> [core 0] -> [thread 0] -> Linux CPU 2 |
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-> [thread 1] -> Linux CPU 6 |
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-> [core 1] -> [thread 0] -> Linux CPU 3 |
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-> [thread 1] -> Linux CPU 7 |
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AMD nomenclature for CMT systems:: |
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[node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0 |
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-> [Compute Unit Core 1] -> Linux CPU 1 |
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-> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2 |
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-> [Compute Unit Core 1] -> Linux CPU 3 |
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[node 1] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 4 |
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-> [Compute Unit Core 1] -> Linux CPU 5 |
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-> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 6 |
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-> [Compute Unit Core 1] -> Linux CPU 7
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