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

[Roman Gushchin <roman.gushchin@linux.dev>]

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>

Thanks!