Root/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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