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222 lines
6.8 KiB
ReStructuredText
222 lines
6.8 KiB
ReStructuredText
.. 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/cputopology.txt. 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 <tglx@linutronix.de> and Borislav Petkov <bp@alien8.de>.
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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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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.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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- 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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