Re: [PATCH v3 1/2] cgroup: Show # of subsystem CSSes in cgroup.stat

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

On Wed, Jul 10, 2024 at 02:23:52PM -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 as long as it is not zero.  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 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 54
> 	nr_dying_descendants 44
> 	nr_cpuset 1
> 	nr_cpu 40
> 	nr_io 40
> 	nr_memory 54
> 	nr_dying_memory 44
> 	nr_perf_event 55
> 	nr_hugetlb 1
> 	nr_pids 54
> 	nr_rdma 1
> 	nr_misc 1
> 
> Another sample output from system.slice/cgroup.stat was:
> 
> 	nr_descendants 32
> 	nr_dying_descendants 37
> 	nr_cpu 30
> 	nr_io 30
> 	nr_memory 32
> 	nr_dying_memory 37
> 	nr_perf_event 33
> 	nr_pids 32
> 
> Signed-off-by: Waiman Long <longman@redhat.com>

I like it way more than the previous version, thank you for the update.

> ---
>  Documentation/admin-guide/cgroup-v2.rst | 14 ++++++-
>  include/linux/cgroup-defs.h             |  7 ++++
>  kernel/cgroup/cgroup.c                  | 50 ++++++++++++++++++++++++-
>  3 files changed, 68 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 52763d6b2919..9031419271cd 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -981,6 +981,16 @@ 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 live cgroups associated with that cgroup
> +		subsystem (e.g. memory) at and beneath the current
> +		cgroup.  An entry will only be shown if it is not zero.
> +
> +	  nr_dying_<cgroup_subsys>
> +		Total number of dying cgroups associated with that cgroup
> +		subsystem (e.g. memory) beneath the current cgroup.
> +		An entry will only be shown if it is not zero.
> +
>    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 +2940,8 @@ 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..62de18874508 100644
> --- a/include/linux/cgroup-defs.h
> +++ b/include/linux/cgroup-defs.h
> @@ -210,6 +210,13 @@ struct cgroup_subsys_state {
>  	 * fields of the containing structure.
>  	 */
>  	struct cgroup_subsys_state *parent;
> +
> +	/*
> +	 * Keep track of total numbers of visible and dying descendant CSSes.
> +	 * Protected by cgroup_mutex.
> +	 */
> +	int nr_descendants;
> +	int nr_dying_descendants;
>  };
>  
>  /*
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index c8e4b62b436a..18c982a06446 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -3669,12 +3669,34 @@ 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_state *css;
> +	int ssid;
>  
> +	/* cgroup_mutex required for for_each_css() */
> +	cgroup_lock();

I *guess* it can be done under a rcu_read_lock(), isn't it?
That would eliminate a need for the second patch as well, which
is questionable (e.g. one unprivileged user can block others?)

Thanks!