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

[Waiman Long]

On 7/10/24 17:43, Roman Gushchin wrote:
> 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@xxxxxxxxxx>
> 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?)

I am just following the instruction in the for_each_css() macro:

 *
 * Should be called under cgroup_mutex.
 */

I think taking rcu_read_lock() should also work in this case. Will try 
it out and update the patch after some testing.

Thanks,
Longman