How CPU topology info is exported via sysfs

CPU topology info is exported via sysfs. Items (attributes) are similarto /proc/cpuinfo output of some architectures. They reside in/sys/devices/system/cpu/cpuX/topology/. Please refer to the ABI file:ABI file 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 willonly be created if an architecture provides the related macros as describedbelow.

For an architecture to support this feature, it must define some ofthese 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**_idmacros is int.The type of**_cpumaskmacros is(const)structcpumask*. The lattercorrespond with appropriate**_siblings sysfs attributes (except fortopology_sibling_cpumask() which corresponds with thread_siblings).

To be consistent on all architectures, include/linux/topology.hprovides default definitions for any of the above macros that arenot 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 internalsource 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 beenHOTPLUGGED off or exceed the limit of CPUs allowed by thekernel 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 bebrought online if they are present. [cpu_possible_mask]

present:

CPUs that have been identified as being present in thesystem. [cpu_present_mask]

The format for the above output is compatible withcpulist_parse()[see <linux/cpumask.h>]. Some examples follow.

In this example, there are 64 CPUs in the system but cpus 32-63 exceedthe kernel max which is limited to 0..31 by the NR_CPUS config optionbeing 32. Note also that CPUs 2 and 4-31 are not online but could bebrought 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 wasstarted with possible_cpus=144. There are 4 CPUs in the system and cpu2was manually taken offline (and is the only CPU that can be broughtonline.):

kernel_max: 127   offline: 2,4-127,128-143    online: 0-1,3  possible: 0-127   present: 0-3

SeeCPU hotplug in the Kernel for the possible_cpus=NUMkernel start parameter as well as more information on the various cpumasks.