1===========================================
2How CPU topology info is exported via sysfs
3===========================================
4
5CPU topology info is exported via sysfs. Items (attributes) are similar
6to /proc/cpuinfo output of some architectures. They reside in
7/sys/devices/system/cpu/cpuX/topology/. Please refer to the ABI file:
8Documentation/ABI/stable/sysfs-devices-system-cpu.
9
10Architecture-neutral, drivers/base/topology.c, exports these attributes.
11However, the book and drawer related sysfs files will only be created if
12CONFIG_SCHED_BOOK and CONFIG_SCHED_DRAWER are selected, respectively.
13
14CONFIG_SCHED_BOOK and CONFIG_SCHED_DRAWER are currently only used on s390,
15where they reflect the cpu and cache hierarchy.
16
17For an architecture to support this feature, it must define some of
18these macros in include/asm-XXX/topology.h::
19
20	#define topology_physical_package_id(cpu)
21	#define topology_die_id(cpu)
22	#define topology_core_id(cpu)
23	#define topology_book_id(cpu)
24	#define topology_drawer_id(cpu)
25	#define topology_sibling_cpumask(cpu)
26	#define topology_core_cpumask(cpu)
27	#define topology_die_cpumask(cpu)
28	#define topology_book_cpumask(cpu)
29	#define topology_drawer_cpumask(cpu)
30
31The type of ``**_id macros`` is int.
32The type of ``**_cpumask macros`` is ``(const) struct cpumask *``. The latter
33correspond with appropriate ``**_siblings`` sysfs attributes (except for
34topology_sibling_cpumask() which corresponds with thread_siblings).
35
36To be consistent on all architectures, include/linux/topology.h
37provides default definitions for any of the above macros that are
38not defined by include/asm-XXX/topology.h:
39
401) topology_physical_package_id: -1
412) topology_die_id: -1
423) topology_core_id: 0
434) topology_sibling_cpumask: just the given CPU
445) topology_core_cpumask: just the given CPU
456) topology_die_cpumask: just the given CPU
46
47For architectures that don't support books (CONFIG_SCHED_BOOK) there are no
48default definitions for topology_book_id() and topology_book_cpumask().
49For architectures that don't support drawers (CONFIG_SCHED_DRAWER) there are
50no default definitions for topology_drawer_id() and topology_drawer_cpumask().
51
52Additionally, CPU topology information is provided under
53/sys/devices/system/cpu and includes these files.  The internal
54source for the output is in brackets ("[]").
55
56    =========== ==========================================================
57    kernel_max: the maximum CPU index allowed by the kernel configuration.
58		[NR_CPUS-1]
59
60    offline:	CPUs that are not online because they have been
61		HOTPLUGGED off or exceed the limit of CPUs allowed by the
62		kernel configuration (kernel_max above).
63		[~cpu_online_mask + cpus >= NR_CPUS]
64
65    online:	CPUs that are online and being scheduled [cpu_online_mask]
66
67    possible:	CPUs that have been allocated resources and can be
68		brought online if they are present. [cpu_possible_mask]
69
70    present:	CPUs that have been identified as being present in the
71		system. [cpu_present_mask]
72    =========== ==========================================================
73
74The format for the above output is compatible with cpulist_parse()
75[see <linux/cpumask.h>].  Some examples follow.
76
77In this example, there are 64 CPUs in the system but cpus 32-63 exceed
78the kernel max which is limited to 0..31 by the NR_CPUS config option
79being 32.  Note also that CPUs 2 and 4-31 are not online but could be
80brought online as they are both present and possible::
81
82     kernel_max: 31
83        offline: 2,4-31,32-63
84         online: 0-1,3
85       possible: 0-31
86        present: 0-31
87
88In this example, the NR_CPUS config option is 128, but the kernel was
89started with possible_cpus=144.  There are 4 CPUs in the system and cpu2
90was manually taken offline (and is the only CPU that can be brought
91online.)::
92
93     kernel_max: 127
94        offline: 2,4-127,128-143
95         online: 0-1,3
96       possible: 0-127
97        present: 0-3
98
99See Documentation/core-api/cpu_hotplug.rst for the possible_cpus=NUM
100kernel start parameter as well as more information on the various cpumasks.
101