Re: [PATCH-cgroup v2] cgroup: Show # of subsystem CSSes in root cgroup.stat

[Kamalesh Babulal]

On 7/9/24 6:58 PM, Waiman Long wrote:
> 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.  This patch adds CSS counts in the cgroup_subsys structure
> to keep track of the number of CSSes for each of the cgroup subsystems.
> 
> As cgroup v2 had deprecated the use of /proc/cgroups, the root
> cgroup.stat file is extended to show the number of outstanding CSSes
> associated with all the non-inhibited cgroup subsystems that have been
> bound to cgroup v2.  This will help us pinpoint which subsystems may be
> responsible for the increasing number of dying (nr_dying_descendants)
> cgroups.
> 
> 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 53
> 	nr_dying_descendants 34
> 	nr_cpuset 1
> 	nr_cpu 40
> 	nr_io 40
> 	nr_memory 87
> 	nr_perf_event 54
> 	nr_hugetlb 1
> 	nr_pids 53
> 	nr_rdma 1
> 	nr_misc 1
> 
> In this particular case, it can be seen that memory cgroup is the most
> likely culprit for causing the 34 dying cgroups.
> 
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 10 ++++++++--
>  include/linux/cgroup-defs.h             |  3 +++
>  kernel/cgroup/cgroup.c                  | 19 +++++++++++++++++++
>  3 files changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 52763d6b2919..65af2f30196f 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -981,6 +981,12 @@ All cgroup core files are prefixed with "cgroup."
>  		A dying cgroup can consume system resources not exceeding
>  		limits, which were active at the moment of cgroup deletion.
>  
> +	  nr_<cgroup_subsys>
> +		Total number of cgroups associated with that cgroup
> +		subsystem, e.g. cpuset or memory.  These cgroup counts
> +		will only be shown in the root cgroup and for subsystems
> +		bound to cgroup v2.
> +
>    cgroup.freeze
>  	A read-write single value file which exists on non-root cgroups.
>  	Allowed values are "0" and "1". The default is "0".
> @@ -2930,8 +2936,8 @@ Deprecated v1 Core Features
>  
>  - "cgroup.clone_children" is removed.
>  
> -- /proc/cgroups is meaningless for v2.  Use "cgroup.controllers" file
> -  at the root instead.
> +- /proc/cgroups is meaningless for v2.  Use "cgroup.controllers" or
> +  "cgroup.stat" files at the root instead.
>  
>  
>  Issues with v1 and Rationales for v2
> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
> index b36690ca0d3f..522ab77f0406 100644
> --- a/include/linux/cgroup-defs.h
> +++ b/include/linux/cgroup-defs.h
> @@ -776,6 +776,9 @@ struct cgroup_subsys {
>  	 * specifies the mask of subsystems that this one depends on.
>  	 */
>  	unsigned int depends_on;
> +
> +	/* Number of CSSes, used only for /proc/cgroups */
> +	atomic_t nr_csses;
>  };
>  
>  extern struct percpu_rw_semaphore cgroup_threadgroup_rwsem;
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index c8e4b62b436a..48eba2737b1a 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -3669,12 +3669,27 @@ static int cgroup_events_show(struct seq_file *seq, void *v)
>  static int cgroup_stat_show(struct seq_file *seq, void *v)
>  {
>  	struct cgroup *cgroup = seq_css(seq)->cgroup;
> +	struct cgroup_subsys *ss;
> +	int i;
>  
>  	seq_printf(seq, "nr_descendants %d\n",
>  		   cgroup->nr_descendants);
>  	seq_printf(seq, "nr_dying_descendants %d\n",
>  		   cgroup->nr_dying_descendants);
>  
> +	if (cgroup_parent(cgroup))
> +		return 0;
> +
> +	/*
> +	 * For the root cgroup, shows the number of csses associated
> +	 * with each of non-inhibited cgroup subsystems bound to it.
> +	 */
> +	do_each_subsys_mask(ss, i, ~cgrp_dfl_inhibit_ss_mask) {
> +		if (ss->root != &cgrp_dfl_root)
> +			continue;
> +		seq_printf(seq, "nr_%s %d\n", ss->name,
> +			   atomic_read(&ss->nr_csses));
> +	} while_each_subsys_mask();
>  	return 0;
>  }
>  

Thanks for adding nr_csses, the patch looks good to me. A preference comment,
nr_<subsys>_css format, makes it easier to interpret the count.

With or without the changes to the cgroup subsys format:

Reviewed-by: Kamalesh Babulal <kamalesh.babulal@xxxxxxxxxx>

-- 
Thanks,
Kamalesh