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d5caec394a
Since kernel commit ff58fa7f55 ("Documentation: Update CPU hotplug and move it to core-api"),
cpu_hotplug.txt has been removed. We should update it in here.
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Link: https://lore.kernel.org/r/1624412269-13155-1-git-send-email-xuyang2018.jy@fujitsu.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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4.0 KiB
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101 lines
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===========================================
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How CPU topology info is exported via sysfs
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===========================================
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CPU topology info is exported via sysfs. Items (attributes) are similar
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to /proc/cpuinfo output of some architectures. They reside in
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/sys/devices/system/cpu/cpuX/topology/. Please refer to the ABI file:
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Documentation/ABI/stable/sysfs-devices-system-cpu.
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Architecture-neutral, drivers/base/topology.c, exports these attributes.
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However, the book and drawer related sysfs files will only be created if
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CONFIG_SCHED_BOOK and CONFIG_SCHED_DRAWER are selected, respectively.
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CONFIG_SCHED_BOOK and CONFIG_SCHED_DRAWER are currently only used on s390,
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where they reflect the cpu and cache hierarchy.
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For an architecture to support this feature, it must define some of
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these macros in include/asm-XXX/topology.h::
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#define topology_physical_package_id(cpu)
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#define topology_die_id(cpu)
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#define topology_core_id(cpu)
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#define topology_book_id(cpu)
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#define topology_drawer_id(cpu)
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#define topology_sibling_cpumask(cpu)
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#define topology_core_cpumask(cpu)
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#define topology_die_cpumask(cpu)
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#define topology_book_cpumask(cpu)
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#define topology_drawer_cpumask(cpu)
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The type of ``**_id macros`` is int.
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The type of ``**_cpumask macros`` is ``(const) struct cpumask *``. The latter
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correspond with appropriate ``**_siblings`` sysfs attributes (except for
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topology_sibling_cpumask() which corresponds with thread_siblings).
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To be consistent on all architectures, include/linux/topology.h
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provides default definitions for any of the above macros that are
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not defined by include/asm-XXX/topology.h:
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1) topology_physical_package_id: -1
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2) topology_die_id: -1
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3) topology_core_id: 0
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4) topology_sibling_cpumask: just the given CPU
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5) topology_core_cpumask: just the given CPU
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6) topology_die_cpumask: just the given CPU
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For architectures that don't support books (CONFIG_SCHED_BOOK) there are no
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default definitions for topology_book_id() and topology_book_cpumask().
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For architectures that don't support drawers (CONFIG_SCHED_DRAWER) there are
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no default definitions for topology_drawer_id() and topology_drawer_cpumask().
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Additionally, CPU topology information is provided under
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/sys/devices/system/cpu and includes these files. The internal
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source for the output is in brackets ("[]").
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=========== ==========================================================
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kernel_max: the maximum CPU index allowed by the kernel configuration.
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[NR_CPUS-1]
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offline: CPUs that are not online because they have been
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HOTPLUGGED off or exceed the limit of CPUs allowed by the
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kernel configuration (kernel_max above).
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[~cpu_online_mask + cpus >= NR_CPUS]
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online: CPUs that are online and being scheduled [cpu_online_mask]
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possible: CPUs that have been allocated resources and can be
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brought online if they are present. [cpu_possible_mask]
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present: CPUs that have been identified as being present in the
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system. [cpu_present_mask]
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=========== ==========================================================
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The format for the above output is compatible with cpulist_parse()
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[see <linux/cpumask.h>]. Some examples follow.
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In this example, there are 64 CPUs in the system but cpus 32-63 exceed
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the kernel max which is limited to 0..31 by the NR_CPUS config option
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being 32. Note also that CPUs 2 and 4-31 are not online but could be
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brought online as they are both present and possible::
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kernel_max: 31
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offline: 2,4-31,32-63
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online: 0-1,3
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possible: 0-31
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present: 0-31
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In this example, the NR_CPUS config option is 128, but the kernel was
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started with possible_cpus=144. There are 4 CPUs in the system and cpu2
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was manually taken offline (and is the only CPU that can be brought
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online.)::
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kernel_max: 127
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offline: 2,4-127,128-143
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online: 0-1,3
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possible: 0-127
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present: 0-3
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See Documentation/core-api/cpu_hotplug.rst for the possible_cpus=NUM
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kernel start parameter as well as more information on the various cpumasks.
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