[RFC] Display the cpu of sched domain in procfs

[Chen Yu]

Problem:
sched domain topology is not always consistent with the CPU topology exposed at
/sys/devices/system/cpu/cpuX/topology,  which makes it
hard for monitor tools to distinguish the CPUs among different sched domains.

For example, on x86 if there are NUMA nodes within a package, say,
SNC(Sub-Numa-Cluster),
then there would be no die sched domain but only NUMA sched domains
created. As a result,
you don't know what the sched domain hierarchical is by only looking
at /sys/devices/system/cpu/cpuX/topology.

Although by appending sched_debug in command line would show the sched
domain CPU topology,
it is only printed once during boot up, which makes it hard to track
at run-time.

Proposal:
Add *span* filed under proc sched_domain directory to represent the
set of CPUs in each
sched domain.

Question:
*Before sending the patch out, may I have you opinions on whether this
is doable?*


Here are the sample output on a SNC system after the patch been applied:
grep . /proc/sys/kernel/sched_domain/cpu0/domain*/span
/proc/sys/kernel/sched_domain/cpu0/domain0/span:0,96     (SMT domain)
/proc/sys/kernel/sched_domain/cpu0/domain1/span:0-3,7-9,13-15,19-20,
(MC domain)

                   96-99,103-105,109-111,

                   115-116
/proc/sys/kernel/sched_domain/cpu0/domain2/span:0-23,96-119
         (NUMA domain)
/proc/sys/kernel/sched_domain/cpu0/domain3/span:0-191
           (NUMA domain)


FYI, the corresponding CPU topology is:
grep . /sys/devices/system/cpu/cpu0/topology/*cpus_list
/sys/devices/system/cpu/cpu0/topology/core_cpus_list:0,96
/sys/devices/system/cpu/cpu0/topology/die_cpus_list:0-23,96-119
/sys/devices/system/cpu/cpu0/topology/package_cpus_list:0-47,96-143


thanks,
Chenyu