linux/Documentation/cputopology.txt
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   1
   2Export CPU topology info via sysfs. Items (attributes) are similar
   3to /proc/cpuinfo.
   4
   51) /sys/devices/system/cpu/cpuX/topology/physical_package_id:
   6
   7        physical package id of cpuX. Typically corresponds to a physical
   8        socket number, but the actual value is architecture and platform
   9        dependent.
  10
  112) /sys/devices/system/cpu/cpuX/topology/core_id:
  12
  13        the CPU core ID of cpuX. Typically it is the hardware platform's
  14        identifier (rather than the kernel's).  The actual value is
  15        architecture and platform dependent.
  16
  173) /sys/devices/system/cpu/cpuX/topology/book_id:
  18
  19        the book ID of cpuX. Typically it is the hardware platform's
  20        identifier (rather than the kernel's).  The actual value is
  21        architecture and platform dependent.
  22
  234) /sys/devices/system/cpu/cpuX/topology/thread_siblings:
  24
  25        internal kernel map of cpuX's hardware threads within the same
  26        core as cpuX
  27
  285) /sys/devices/system/cpu/cpuX/topology/core_siblings:
  29
  30        internal kernel map of cpuX's hardware threads within the same
  31        physical_package_id.
  32
  336) /sys/devices/system/cpu/cpuX/topology/book_siblings:
  34
  35        internal kernel map of cpuX's hardware threads within the same
  36        book_id.
  37
  38To implement it in an architecture-neutral way, a new source file,
  39drivers/base/topology.c, is to export the 4 or 6 attributes. The two book
  40related sysfs files will only be created if CONFIG_SCHED_BOOK is selected.
  41
  42For an architecture to support this feature, it must define some of
  43these macros in include/asm-XXX/topology.h:
  44#define topology_physical_package_id(cpu)
  45#define topology_core_id(cpu)
  46#define topology_book_id(cpu)
  47#define topology_thread_cpumask(cpu)
  48#define topology_core_cpumask(cpu)
  49#define topology_book_cpumask(cpu)
  50
  51The type of **_id is int.
  52The type of siblings is (const) struct cpumask *.
  53
  54To be consistent on all architectures, include/linux/topology.h
  55provides default definitions for any of the above macros that are
  56not defined by include/asm-XXX/topology.h:
  571) physical_package_id: -1
  582) core_id: 0
  593) thread_siblings: just the given CPU
  604) core_siblings: just the given CPU
  61
  62For architectures that don't support books (CONFIG_SCHED_BOOK) there are no
  63default definitions for topology_book_id() and topology_book_cpumask().
  64
  65Additionally, CPU topology information is provided under
  66/sys/devices/system/cpu and includes these files.  The internal
  67source for the output is in brackets ("[]").
  68
  69    kernel_max: the maximum CPU index allowed by the kernel configuration.
  70                [NR_CPUS-1]
  71
  72    offline:    CPUs that are not online because they have been
  73                HOTPLUGGED off (see cpu-hotplug.txt) or exceed the limit
  74                of CPUs allowed by the kernel configuration (kernel_max
  75                above). [~cpu_online_mask + cpus >= NR_CPUS]
  76
  77    online:     CPUs that are online and being scheduled [cpu_online_mask]
  78
  79    possible:   CPUs that have been allocated resources and can be
  80                brought online if they are present. [cpu_possible_mask]
  81
  82    present:    CPUs that have been identified as being present in the
  83                system. [cpu_present_mask]
  84
  85The format for the above output is compatible with cpulist_parse()
  86[see <linux/cpumask.h>].  Some examples follow.
  87
  88In this example, there are 64 CPUs in the system but cpus 32-63 exceed
  89the kernel max which is limited to 0..31 by the NR_CPUS config option
  90being 32.  Note also that CPUs 2 and 4-31 are not online but could be
  91brought online as they are both present and possible.
  92
  93     kernel_max: 31
  94        offline: 2,4-31,32-63
  95         online: 0-1,3
  96       possible: 0-31
  97        present: 0-31
  98
  99In this example, the NR_CPUS config option is 128, but the kernel was
 100started with possible_cpus=144.  There are 4 CPUs in the system and cpu2
 101was manually taken offline (and is the only CPU that can be brought
 102online.)
 103
 104     kernel_max: 127
 105        offline: 2,4-127,128-143
 106         online: 0-1,3
 107       possible: 0-127
 108        present: 0-3
 109
 110See cpu-hotplug.txt for the possible_cpus=NUM kernel start parameter
 111as well as more information on the various cpumasks.
 112
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