Re: [PATCH-cgroup v7] cgroup: Show # of subsystem CSSes in cgroup.stat

[Waiman Long <longman@redhat.com>]

On 7/15/24 13:30, Roman Gushchin wrote:
> On Mon, Jul 15, 2024 at 11:00:34AM -0400, Waiman Long wrote:
>> Cgroup subsystem state (CSS) is an abstraction in the cgroup layer to
>> help manage different structures in various cgroup subsystems by being
>> an embedded element inside a larger structure like cpuset or mem_cgroup.
>>
>> The /proc/cgroups file shows the number of cgroups for each of the
>> subsystems.  With cgroup v1, the number of CSSes is the same as the
>> number of cgroups.  That is not the case anymore with cgroup v2. The
>> /proc/cgroups file cannot show the actual number of CSSes for the
>> subsystems that are bound to cgroup v2.
>>
>> So if a v2 cgroup subsystem is leaking cgroups (usually memory cgroup),
>> we can't tell by looking at /proc/cgroups which cgroup subsystems may
>> be responsible.
>>
>> As cgroup v2 had deprecated the use of /proc/cgroups, the hierarchical
>> cgroup.stat file is now being extended to show the number of live and
>> dying CSSes associated with all the non-inhibited cgroup subsystems that
>> have been bound to cgroup v2. The number includes CSSes in the current
>> cgroup as well as in all the descendants underneath it.  This will help
>> us pinpoint which subsystems are responsible for the increasing number
>> of dying (nr_dying_descendants) cgroups.
>>
>> The CSSes dying counts are stored in the cgroup structure itself
>> instead of inside the CSS as suggested by Johannes. This will allow
>> us to accurately track dying counts of cgroup subsystems that have
>> recently been disabled in a cgroup. It is now possible that a zero
>> subsystem number is coupled with a non-zero dying subsystem number.
>>
>> The cgroup-v2.rst file is updated to discuss this new behavior.
>>
>> With this patch applied, a sample output from root cgroup.stat file
>> was shown below.
>>
>> 	nr_descendants 56
>> 	nr_subsys_cpuset 1
>> 	nr_subsys_cpu 43
>> 	nr_subsys_io 43
>> 	nr_subsys_memory 56
>> 	nr_subsys_perf_event 57
>> 	nr_subsys_hugetlb 1
>> 	nr_subsys_pids 56
>> 	nr_subsys_rdma 1
>> 	nr_subsys_misc 1
>> 	nr_dying_descendants 30
>> 	nr_dying_subsys_cpuset 0
>> 	nr_dying_subsys_cpu 0
>> 	nr_dying_subsys_io 0
>> 	nr_dying_subsys_memory 30
>> 	nr_dying_subsys_perf_event 0
>> 	nr_dying_subsys_hugetlb 0
>> 	nr_dying_subsys_pids 0
>> 	nr_dying_subsys_rdma 0
>> 	nr_dying_subsys_misc 0
>>
>> Another sample output from system.slice/cgroup.stat was:
>>
>> 	nr_descendants 34
>> 	nr_subsys_cpuset 0
>> 	nr_subsys_cpu 32
>> 	nr_subsys_io 32
>> 	nr_subsys_memory 34
>> 	nr_subsys_perf_event 35
>> 	nr_subsys_hugetlb 0
>> 	nr_subsys_pids 34
>> 	nr_subsys_rdma 0
>> 	nr_subsys_misc 0
>> 	nr_dying_descendants 30
>> 	nr_dying_subsys_cpuset 0
>> 	nr_dying_subsys_cpu 0
>> 	nr_dying_subsys_io 0
>> 	nr_dying_subsys_memory 30
>> 	nr_dying_subsys_perf_event 0
>> 	nr_dying_subsys_hugetlb 0
>> 	nr_dying_subsys_pids 0
>> 	nr_dying_subsys_rdma 0
>> 	nr_dying_subsys_misc 0
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
> Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

Tejun, is this patch ready to be merged or do you have other suggestion 
you have in mind?

Thanks,
Longman