[RFC] Display the cpu of sched domain in procfs

[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

Re: [RFC] Display the cpu of sched domain in procfs

[Valentin Schneider]

Hi,

On 19/02/2020 07:15, Chen Yu wrote:
> 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.
> 

It should (and in my experience, is) be printed any time there is a sched
domain update - hotplug, cpusets, IOW not just at bootup.

e.g. if I hotplug out a CPU:

root@valsch-juno:~# echo 0 > /sys/devices/system/cpu/cpu3/online
[40150.882586] CPU3: shutdown
[40150.885383] psci: CPU3 killed (polled 0 ms)
[40150.891362] CPU0 attaching NULL sched-domain.
[40150.895954] CPU1 attaching NULL sched-domain.
[40150.900433] CPU2 attaching NULL sched-domain.
[40150.906583] CPU3 attaching NULL sched-domain.
[40150.910998] CPU4 attaching NULL sched-domain.
[40150.915444] CPU5 attaching NULL sched-domain.
[40150.920108] CPU0 attaching sched-domain(s):
[40150.924396]  domain-0: span=0,4-5 level=MC
[40150.928592]   groups: 0:{ span=0 cap=444 }, 4:{ span=4 cap=445 }, 5:{ span=5 cap=446 }
[40150.936684]   domain-1: span=0-2,4-5 level=DIE
[40150.941207]    groups: 0:{ span=0,4-5 cap=1335 }, 1:{ span=1-2 cap=2041 }
[40150.948107] CPU1 attaching sched-domain(s):
[40150.952342]  domain-0: span=1-2 level=MC
[40150.956311]   groups: 1:{ span=1 cap=1020 }, 2:{ span=2 cap=1021 }
[40150.962592]   domain-1: span=0-2,4-5 level=DIE
[40150.967082]    groups: 1:{ span=1-2 cap=2041 }, 0:{ span=0,4-5 cap=1335 }
[40150.973984] CPU2 attaching sched-domain(s):
[40150.978208]  domain-0: span=1-2 level=MC
[40150.982176]   groups: 2:{ span=2 cap=1021 }, 1:{ span=1 cap=1021 }
[40150.988431]   domain-1: span=0-2,4-5 level=DIE
[40150.992922]    groups: 1:{ span=1-2 cap=2042 }, 0:{ span=0,4-5 cap=1335 }
[40150.999819] CPU4 attaching sched-domain(s):
[40151.004045]  domain-0: span=0,4-5 level=MC
[40151.008186]   groups: 4:{ span=4 cap=445 }, 5:{ span=5 cap=446 }, 0:{ span=0 cap=444 }
[40151.016220]   domain-1: span=0-2,4-5 level=DIE
[40151.020722]    groups: 0:{ span=0,4-5 cap=1335 }, 1:{ span=1-2 cap=2044 }
[40151.027619] CPU5 attaching sched-domain(s):
[40151.031843]  domain-0: span=0,4-5 level=MC
[40151.035985]   groups: 5:{ span=5 cap=446 }, 0:{ span=0 cap=444 }, 4:{ span=4 cap=445 }
[40151.044021]   domain-1: span=0-2,4-5 level=DIE
[40151.048512]    groups: 0:{ span=0,4-5 cap=1335 }, 1:{ span=1-2 cap=2043 }
[40151.055440] root domain span: 0-2,4-5 (max cpu_capacity = 1024)


Same for setting up cpusets:

root@valsch-juno:~# cgset -r cpuset.mems=0 asym
root@valsch-juno:~# cgset -r cpuset.cpu_exclusive=1 asym
root@valsch-juno:~# 
root@valsch-juno:~# cgcreate -g cpuset:smp
root@valsch-juno:~# cgset -r cpuset.cpus=4-5 smp
root@valsch-juno:~# cgset -r cpuset.mems=0 smp
root@valsch-juno:~# cgset -r cpuset.cpu_exclusive=1 smp
root@valsch-juno:~# 
root@valsch-juno:~# cgset -r cpuset.sched_load_balance=0 .
[40224.135466] CPU0 attaching NULL sched-domain.
[40224.140038] CPU1 attaching NULL sched-domain.
[40224.144531] CPU2 attaching NULL sched-domain.
[40224.148951] CPU3 attaching NULL sched-domain.
[40224.153366] CPU4 attaching NULL sched-domain.
[40224.157811] CPU5 attaching NULL sched-domain.
[40224.162394] CPU0 attaching sched-domain(s):
[40224.166623]  domain-0: span=0,3 level=MC
[40224.170624]   groups: 0:{ span=0 cap=445 }, 3:{ span=3 cap=446 }
[40224.176709]   domain-1: span=0-3 level=DIE
[40224.180884]    groups: 0:{ span=0,3 cap=891 }, 1:{ span=1-2 cap=2044 }
[40224.187497] CPU1 attaching sched-domain(s):
[40224.191753]  domain-0: span=1-2 level=MC
[40224.195724]   groups: 1:{ span=1 cap=1021 }, 2:{ span=2 cap=1023 }
[40224.202010]   domain-1: span=0-3 level=DIE
[40224.206154]    groups: 1:{ span=1-2 cap=2044 }, 0:{ span=0,3 cap=890 }
[40224.212792] CPU2 attaching sched-domain(s):
[40224.217020]  domain-0: span=1-2 level=MC
[40224.220989]   groups: 2:{ span=2 cap=1023 }, 1:{ span=1 cap=1020 }
[40224.227244]   domain-1: span=0-3 level=DIE
[40224.231386]    groups: 1:{ span=1-2 cap=2042 }, 0:{ span=0,3 cap=889 }
[40224.238025] CPU3 attaching sched-domain(s):
[40224.242252]  domain-0: span=0,3 level=MC
[40224.246221]   groups: 3:{ span=3 cap=446 }, 0:{ span=0 cap=443 }
[40224.252329]   domain-1: span=0-3 level=DIE
[40224.256474]    groups: 0:{ span=0,3 cap=889 }, 1:{ span=1-2 cap=2042 }
[40224.263142] root domain span: 0-3 (max cpu_capacity = 1024)
[40224.268945] CPU4 attaching sched-domain(s):
[40224.273200]  domain-0: span=4-5 level=MC
[40224.277173]   groups: 4:{ span=4 cap=446 }, 5:{ span=5 cap=444 }
[40224.283291] CPU5 attaching sched-domain(s):
[40224.287517]  domain-0: span=4-5 level=MC
[40224.291487]   groups: 5:{ span=5 cap=444 }, 4:{ span=4 cap=446 }
[40224.297584] root domain span: 4-5 (max cpu_capacity = 446)
[40224.303185] rd 4-5: CPUs do not have asymmetric capacities

So in short, if you have sched_debug enabled, whatever is reported as the
last sched domain hierarchy in dmesg will be the one in use.

Now, if you have a userspace that tries to be clever and wants to use this
information then yes, this isn't ideal, but then that's a different matter.
I think exposing the NUMA boundaries is fair game - and they already are
via /sys/devices/system/node/node*/. I'm not sure we'd want to expose more
(e.g. MC span), ideally that is something you shouldn't really have to care
about - that's the scheduler's job.

> 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
> 

Re: [RFC] Display the cpu of sched domain in procfs

[Dietmar Eggemann]

On 19/02/2020 09:13, Valentin Schneider wrote:
> Hi,
> 
> On 19/02/2020 07:15, Chen Yu wrote:
>> 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.

What about /proc/schedstat?

E.g. on Intel Xeon CPU E5-2690 v2

$ cat /proc/schedstat | head
version 15
timestamp 4486170100
cpu0 0 0 0 0 0 0 59501267037720 16902762382193 1319621004
domain0 00,00100001 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
domain1 00,3ff003ff 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
domain2 ff,ffffffff 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

        ^^^^^^^^^^^

cpu1 0 0 0 0 0 0 56045879920164 16758983055410 1318489275
domain0 00,00200002 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
domain1 00,3ff003ff 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
domain2 ff,ffffffff 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
...

Re: [RFC] Display the cpu of sched domain in procfs

[Chen Yu]

Hi Valentin,
Thanks very much for looking at my question,
On Wed, Feb 19, 2020 at 4:13 PM Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
> Hi,
>
> On 19/02/2020 07:15, Chen Yu wrote:
> > 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.
> >
>
> It should (and in my experience, is) be printed any time there is a sched
> domain update - hotplug, cpusets, IOW not just at bootup.
>
Right, whenever domain has changed it will be printed.
> e.g. if I hotplug out a CPU:
>
> root@valsch-juno:~# echo 0 > /sys/devices/system/cpu/cpu3/online
> [40150.882586] CPU3: shutdown
> [40150.885383] psci: CPU3 killed (polled 0 ms)
> [40150.891362] CPU0 attaching NULL sched-domain.
> [40150.895954] CPU1 attaching NULL sched-domain.
> [40150.900433] CPU2 attaching NULL sched-domain.
> [40150.906583] CPU3 attaching NULL sched-domain.
> [40150.910998] CPU4 attaching NULL sched-domain.
> [40150.915444] CPU5 attaching NULL sched-domain.
> [40150.920108] CPU0 attaching sched-domain(s):
> [40150.924396]  domain-0: span=0,4-5 level=MC
> [40150.928592]   groups: 0:{ span=0 cap=444 }, 4:{ span=4 cap=445 }, 5:{ span=5 cap=446 }
> [40150.936684]   domain-1: span=0-2,4-5 level=DIE
> [40150.941207]    groups: 0:{ span=0,4-5 cap=1335 }, 1:{ span=1-2 cap=2041 }
> [40150.948107] CPU1 attaching sched-domain(s):
> [40150.952342]  domain-0: span=1-2 level=MC
> [40150.956311]   groups: 1:{ span=1 cap=1020 }, 2:{ span=2 cap=1021 }
> [40150.962592]   domain-1: span=0-2,4-5 level=DIE
> [40150.967082]    groups: 1:{ span=1-2 cap=2041 }, 0:{ span=0,4-5 cap=1335 }
> [40150.973984] CPU2 attaching sched-domain(s):
> [40150.978208]  domain-0: span=1-2 level=MC
> [40150.982176]   groups: 2:{ span=2 cap=1021 }, 1:{ span=1 cap=1021 }
> [40150.988431]   domain-1: span=0-2,4-5 level=DIE
> [40150.992922]    groups: 1:{ span=1-2 cap=2042 }, 0:{ span=0,4-5 cap=1335 }
> [40150.999819] CPU4 attaching sched-domain(s):
> [40151.004045]  domain-0: span=0,4-5 level=MC
> [40151.008186]   groups: 4:{ span=4 cap=445 }, 5:{ span=5 cap=446 }, 0:{ span=0 cap=444 }
> [40151.016220]   domain-1: span=0-2,4-5 level=DIE
> [40151.020722]    groups: 0:{ span=0,4-5 cap=1335 }, 1:{ span=1-2 cap=2044 }
> [40151.027619] CPU5 attaching sched-domain(s):
> [40151.031843]  domain-0: span=0,4-5 level=MC
> [40151.035985]   groups: 5:{ span=5 cap=446 }, 0:{ span=0 cap=444 }, 4:{ span=4 cap=445 }
> [40151.044021]   domain-1: span=0-2,4-5 level=DIE
> [40151.048512]    groups: 0:{ span=0,4-5 cap=1335 }, 1:{ span=1-2 cap=2043 }
> [40151.055440] root domain span: 0-2,4-5 (max cpu_capacity = 1024)
>
>
> Same for setting up cpusets:
>
> root@valsch-juno:~# cgset -r cpuset.mems=0 asym
> root@valsch-juno:~# cgset -r cpuset.cpu_exclusive=1 asym
> root@valsch-juno:~#
> root@valsch-juno:~# cgcreate -g cpuset:smp
> root@valsch-juno:~# cgset -r cpuset.cpus=4-5 smp
> root@valsch-juno:~# cgset -r cpuset.mems=0 smp
> root@valsch-juno:~# cgset -r cpuset.cpu_exclusive=1 smp
> root@valsch-juno:~#
> root@valsch-juno:~# cgset -r cpuset.sched_load_balance=0 .
> [40224.135466] CPU0 attaching NULL sched-domain.
> [40224.140038] CPU1 attaching NULL sched-domain.
> [40224.144531] CPU2 attaching NULL sched-domain.
> [40224.148951] CPU3 attaching NULL sched-domain.
> [40224.153366] CPU4 attaching NULL sched-domain.
> [40224.157811] CPU5 attaching NULL sched-domain.
> [40224.162394] CPU0 attaching sched-domain(s):
> [40224.166623]  domain-0: span=0,3 level=MC
> [40224.170624]   groups: 0:{ span=0 cap=445 }, 3:{ span=3 cap=446 }
> [40224.176709]   domain-1: span=0-3 level=DIE
> [40224.180884]    groups: 0:{ span=0,3 cap=891 }, 1:{ span=1-2 cap=2044 }
> [40224.187497] CPU1 attaching sched-domain(s):
> [40224.191753]  domain-0: span=1-2 level=MC
> [40224.195724]   groups: 1:{ span=1 cap=1021 }, 2:{ span=2 cap=1023 }
> [40224.202010]   domain-1: span=0-3 level=DIE
> [40224.206154]    groups: 1:{ span=1-2 cap=2044 }, 0:{ span=0,3 cap=890 }
> [40224.212792] CPU2 attaching sched-domain(s):
> [40224.217020]  domain-0: span=1-2 level=MC
> [40224.220989]   groups: 2:{ span=2 cap=1023 }, 1:{ span=1 cap=1020 }
> [40224.227244]   domain-1: span=0-3 level=DIE
> [40224.231386]    groups: 1:{ span=1-2 cap=2042 }, 0:{ span=0,3 cap=889 }
> [40224.238025] CPU3 attaching sched-domain(s):
> [40224.242252]  domain-0: span=0,3 level=MC
> [40224.246221]   groups: 3:{ span=3 cap=446 }, 0:{ span=0 cap=443 }
> [40224.252329]   domain-1: span=0-3 level=DIE
> [40224.256474]    groups: 0:{ span=0,3 cap=889 }, 1:{ span=1-2 cap=2042 }
> [40224.263142] root domain span: 0-3 (max cpu_capacity = 1024)
> [40224.268945] CPU4 attaching sched-domain(s):
> [40224.273200]  domain-0: span=4-5 level=MC
> [40224.277173]   groups: 4:{ span=4 cap=446 }, 5:{ span=5 cap=444 }
> [40224.283291] CPU5 attaching sched-domain(s):
> [40224.287517]  domain-0: span=4-5 level=MC
> [40224.291487]   groups: 5:{ span=5 cap=444 }, 4:{ span=4 cap=446 }
> [40224.297584] root domain span: 4-5 (max cpu_capacity = 446)
> [40224.303185] rd 4-5: CPUs do not have asymmetric capacities
>
> So in short, if you have sched_debug enabled, whatever is reported as the
> last sched domain hierarchy in dmesg will be the one in use.
>
> Now, if you have a userspace that tries to be clever and wants to use this
> information then yes, this isn't ideal, but then that's a different matter.
The dmesg might be lost if someone has once used dmesg -c to clear the log,
and the /var/log/dmesg might not always there. And it is not common to trigger
sched domain update once boot up in some environment.
But anyway, this information printed by sched_debug is very fertile for knowing
the topology.
> I think exposing the NUMA boundaries is fair game - and they already are
> via /sys/devices/system/node/node*/.
It seems that the numa sysfs could not reflect the SNC topology, it just has the
*leaf* numa node information. Say, node0 and node1 might form one sched_domain.
> I'm not sure we'd want to expose more
> (e.g. MC span), ideally that is something you shouldn't really have to care
> about - that's the scheduler's job.
I agree, it just aims to facilitate the user space, or just for
debugging purpose.
But as Dietmar replied, the /proc/schedstat has already exposed it.


thanks again,
Chenyu

Re: [RFC] Display the cpu of sched domain in procfs

[Chen Yu]

Hi Dietmar,

On Wed, Feb 19, 2020 at 4:53 PM Dietmar Eggemann
<dietmar.eggemann@arm.com> wrote:
>
> On 19/02/2020 09:13, Valentin Schneider wrote:
> > Hi,
> >
> > On 19/02/2020 07:15, Chen Yu wrote:
> >> 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.
>
> What about /proc/schedstat?
>
That's it!  I did not notice it before, this should work although the
user space might
need to parse the format.


-- 
Thanks,
Chenyu
> E.g. on Intel Xeon CPU E5-2690 v2
>
> $ cat /proc/schedstat | head
> version 15
> timestamp 4486170100
> cpu0 0 0 0 0 0 0 59501267037720 16902762382193 1319621004
> domain0 00,00100001 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> domain1 00,3ff003ff 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> domain2 ff,ffffffff 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
>
>         ^^^^^^^^^^^
>
> cpu1 0 0 0 0 0 0 56045879920164 16758983055410 1318489275
> domain0 00,00200002 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> domain1 00,3ff003ff 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> domain2 ff,ffffffff 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> ...
>

Re: [RFC] Display the cpu of sched domain in procfs

[Valentin Schneider]

On 19/02/2020 10:00, Chen Yu wrote:
>> Now, if you have a userspace that tries to be clever and wants to use this
>> information then yes, this isn't ideal, but then that's a different matter.
> The dmesg might be lost if someone has once used dmesg -c to clear the log,
> and the /var/log/dmesg might not always there. And it is not common to trigger
> sched domain update once boot up in some environment.
> But anyway, this information printed by sched_debug is very fertile for knowing
> the topology.
>> I think exposing the NUMA boundaries is fair game - and they already are
>> via /sys/devices/system/node/node*/.
> It seems that the numa sysfs could not reflect the SNC topology, it just has the
> *leaf* numa node information. Say, node0 and node1 might form one sched_domain.

Right, but if you have leaves + distance table, then userspace can try to
be clever about it without being exposed to scheduler innards.