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Documentation / cputopology.txt




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Based on kernel version 4.3. Page generated on 2015-11-02 12:44 EST.

1	
2	Export CPU topology info via sysfs. Items (attributes) are similar
3	to /proc/cpuinfo output of some architectures:
4	
5	1) /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	
11	2) /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	
17	3) /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	
23	4) /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	
28	5) /sys/devices/system/cpu/cpuX/topology/thread_siblings_list:
29	
30		human-readable list of cpuX's hardware threads within the same
31		core as cpuX.
32	
33	6) /sys/devices/system/cpu/cpuX/topology/core_siblings:
34	
35		internal kernel map of cpuX's hardware threads within the same
36		physical_package_id.
37	
38	7) /sys/devices/system/cpu/cpuX/topology/core_siblings_list:
39	
40		human-readable list of cpuX's hardware threads within the same
41		physical_package_id.
42	
43	8) /sys/devices/system/cpu/cpuX/topology/book_siblings:
44	
45		internal kernel map of cpuX's hardware threads within the same
46		book_id.
47	
48	9) /sys/devices/system/cpu/cpuX/topology/book_siblings_list:
49	
50		human-readable list of cpuX's hardware threads within the same
51		book_id.
52	
53	To implement it in an architecture-neutral way, a new source file,
54	drivers/base/topology.c, is to export the 6 or 9 attributes. The three book
55	related sysfs files will only be created if CONFIG_SCHED_BOOK is selected.
56	
57	For an architecture to support this feature, it must define some of
58	these macros in include/asm-XXX/topology.h:
59	#define topology_physical_package_id(cpu)
60	#define topology_core_id(cpu)
61	#define topology_book_id(cpu)
62	#define topology_sibling_cpumask(cpu)
63	#define topology_core_cpumask(cpu)
64	#define topology_book_cpumask(cpu)
65	
66	The type of **_id macros is int.
67	The type of **_cpumask macros is (const) struct cpumask *. The latter
68	correspond with appropriate **_siblings sysfs attributes (except for
69	topology_sibling_cpumask() which corresponds with thread_siblings).
70	
71	To be consistent on all architectures, include/linux/topology.h
72	provides default definitions for any of the above macros that are
73	not defined by include/asm-XXX/topology.h:
74	1) physical_package_id: -1
75	2) core_id: 0
76	3) sibling_cpumask: just the given CPU
77	4) core_cpumask: just the given CPU
78	
79	For architectures that don't support books (CONFIG_SCHED_BOOK) there are no
80	default definitions for topology_book_id() and topology_book_cpumask().
81	
82	Additionally, CPU topology information is provided under
83	/sys/devices/system/cpu and includes these files.  The internal
84	source for the output is in brackets ("[]").
85	
86	    kernel_max: the maximum CPU index allowed by the kernel configuration.
87			[NR_CPUS-1]
88	
89	    offline:	CPUs that are not online because they have been
90			HOTPLUGGED off (see cpu-hotplug.txt) or exceed the limit
91			of CPUs allowed by the kernel configuration (kernel_max
92			above). [~cpu_online_mask + cpus >= NR_CPUS]
93	
94	    online:	CPUs that are online and being scheduled [cpu_online_mask]
95	
96	    possible:	CPUs that have been allocated resources and can be
97			brought online if they are present. [cpu_possible_mask]
98	
99	    present:	CPUs that have been identified as being present in the
100			system. [cpu_present_mask]
101	
102	The format for the above output is compatible with cpulist_parse()
103	[see <linux/cpumask.h>].  Some examples follow.
104	
105	In this example, there are 64 CPUs in the system but cpus 32-63 exceed
106	the kernel max which is limited to 0..31 by the NR_CPUS config option
107	being 32.  Note also that CPUs 2 and 4-31 are not online but could be
108	brought online as they are both present and possible.
109	
110	     kernel_max: 31
111	        offline: 2,4-31,32-63
112	         online: 0-1,3
113	       possible: 0-31
114	        present: 0-31
115	
116	In this example, the NR_CPUS config option is 128, but the kernel was
117	started with possible_cpus=144.  There are 4 CPUs in the system and cpu2
118	was manually taken offline (and is the only CPU that can be brought
119	online.)
120	
121	     kernel_max: 127
122	        offline: 2,4-127,128-143
123	         online: 0-1,3
124	       possible: 0-127
125	        present: 0-3
126	
127	See cpu-hotplug.txt for the possible_cpus=NUM kernel start parameter
128	as well as more information on the various cpumasks.
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