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

Roman Gushchin
2024-07-10T21:44:01Z

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!