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diff --git a/Documentation/admin-guide/cputopology.rst b/Documentation/admin-guide/cputopology.rst new file mode 100644 index 000000000..d29cacc9b --- /dev/null +++ b/Documentation/admin-guide/cputopology.rst @@ -0,0 +1,101 @@ +=========================================== +How CPU topology info is exported via sysfs +=========================================== + +CPU topology info is exported via sysfs. Items (attributes) are similar +to /proc/cpuinfo output of some architectures. They reside in +/sys/devices/system/cpu/cpuX/topology/. Please refer to the ABI file: +Documentation/ABI/stable/sysfs-devices-system-cpu. + +Architecture-neutral, drivers/base/topology.c, exports these attributes. +However the die, cluster, book, and drawer hierarchy related sysfs files will +only be created if an architecture provides the related macros as described +below. + +For an architecture to support this feature, it must define some of +these macros in include/asm-XXX/topology.h:: + + #define topology_physical_package_id(cpu) + #define topology_die_id(cpu) + #define topology_cluster_id(cpu) + #define topology_core_id(cpu) + #define topology_book_id(cpu) + #define topology_drawer_id(cpu) + #define topology_sibling_cpumask(cpu) + #define topology_core_cpumask(cpu) + #define topology_cluster_cpumask(cpu) + #define topology_die_cpumask(cpu) + #define topology_book_cpumask(cpu) + #define topology_drawer_cpumask(cpu) + +The type of ``**_id macros`` is int. +The type of ``**_cpumask macros`` is ``(const) struct cpumask *``. The latter +correspond with appropriate ``**_siblings`` sysfs attributes (except for +topology_sibling_cpumask() which corresponds with thread_siblings). + +To be consistent on all architectures, include/linux/topology.h +provides default definitions for any of the above macros that are +not defined by include/asm-XXX/topology.h: + +1) topology_physical_package_id: -1 +2) topology_die_id: -1 +3) topology_cluster_id: -1 +4) topology_core_id: 0 +5) topology_book_id: -1 +6) topology_drawer_id: -1 +7) topology_sibling_cpumask: just the given CPU +8) topology_core_cpumask: just the given CPU +9) topology_cluster_cpumask: just the given CPU +10) topology_die_cpumask: just the given CPU +11) topology_book_cpumask: just the given CPU +12) topology_drawer_cpumask: just the given CPU + +Additionally, CPU topology information is provided under +/sys/devices/system/cpu and includes these files. The internal +source for the output is in brackets ("[]"). + + =========== ========================================================== + kernel_max: the maximum CPU index allowed by the kernel configuration. + [NR_CPUS-1] + + offline: CPUs that are not online because they have been + HOTPLUGGED off or exceed the limit of CPUs allowed by the + kernel configuration (kernel_max above). + [~cpu_online_mask + cpus >= NR_CPUS] + + online: CPUs that are online and being scheduled [cpu_online_mask] + + possible: CPUs that have been allocated resources and can be + brought online if they are present. [cpu_possible_mask] + + present: CPUs that have been identified as being present in the + system. [cpu_present_mask] + =========== ========================================================== + +The format for the above output is compatible with cpulist_parse() +[see <linux/cpumask.h>]. Some examples follow. + +In this example, there are 64 CPUs in the system but cpus 32-63 exceed +the kernel max which is limited to 0..31 by the NR_CPUS config option +being 32. Note also that CPUs 2 and 4-31 are not online but could be +brought online as they are both present and possible:: + + kernel_max: 31 + offline: 2,4-31,32-63 + online: 0-1,3 + possible: 0-31 + present: 0-31 + +In this example, the NR_CPUS config option is 128, but the kernel was +started with possible_cpus=144. There are 4 CPUs in the system and cpu2 +was manually taken offline (and is the only CPU that can be brought +online.):: + + kernel_max: 127 + offline: 2,4-127,128-143 + online: 0-1,3 + possible: 0-127 + present: 0-3 + +See Documentation/core-api/cpu_hotplug.rst for the possible_cpus=NUM +kernel start parameter as well as more information on the various cpumasks. |