Re: [PATCH-cgroup v4] cgroup: Show # of subsystem CSSes in cgroup.stat

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

On Wed, Jul 10, 2024 at 10:51:53PM -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 55
> 	nr_dying_descendants 35
> 	nr_subsys_cpuset 1
> 	nr_subsys_cpu 40
> 	nr_subsys_io 40
> 	nr_subsys_memory 55
> 	nr_dying_subsys_memory 35
> 	nr_subsys_perf_event 56
> 	nr_subsys_hugetlb 1
> 	nr_subsys_pids 55
> 	nr_subsys_rdma 1
> 	nr_subsys_misc 1
> 
> Another sample output from system.slice/cgroup.stat was:
> 
> 	nr_descendants 32
> 	nr_dying_descendants 33
> 	nr_subsys_cpu 30
> 	nr_subsys_io 30
> 	nr_subsys_memory 32
> 	nr_dying_subsys_memory 33
> 	nr_subsys_perf_event 33
> 	nr_subsys_pids 32
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 14 ++++++-
>  include/linux/cgroup-defs.h             |  7 ++++
>  kernel/cgroup/cgroup.c                  | 52 ++++++++++++++++++++++++-
>  3 files changed, 70 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 52763d6b2919..356cd430c888 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_subsys_<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_subsys_<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..cf4fc1c109e2 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -3669,12 +3669,36 @@ 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;
>  
>  	seq_printf(seq, "nr_descendants %d\n",
>  		   cgroup->nr_descendants);
>  	seq_printf(seq, "nr_dying_descendants %d\n",
>  		   cgroup->nr_dying_descendants);
>  
> +	/*
> +	 * Show the number of live and dying csses associated with each of
> +	 * non-inhibited cgroup subsystems bound to cgroup v2 if non-zero.
> +	 *
> +	 * Without proper lock protection, racing is possible. So the
> +	 * numbers may not be consistent when that happens.
> +	 */
> +	rcu_read_lock();
> +	for_each_css(css, ssid, cgroup) {
> +		if ((BIT(ssid) & cgrp_dfl_inhibit_ss_mask) ||
> +		    (cgroup_subsys[ssid]->root !=  &cgrp_dfl_root))
> +			continue;
> +
> +		seq_printf(seq, "nr_subsys_%s %d\n", cgroup_subsys[ssid]->name,
> +			   css->nr_descendants + 1);
> +		/* Current css is online */
> +		if (css->nr_dying_descendants)
> +			seq_printf(seq, "nr_dying_subsys_%s %d\n",
> +				   cgroup_subsys[ssid]->name,
> +				   css->nr_dying_descendants);
> +	}
> +	rcu_read_unlock();
>  	return 0;
>  }
>  
> @@ -5424,6 +5448,8 @@ static void css_release_work_fn(struct work_struct *work)
>  	list_del_rcu(&css->sibling);
>  
>  	if (ss) {
> +		struct cgroup_subsys_state *parent_css;
> +
>  		/* css release path */
>  		if (!list_empty(&css->rstat_css_node)) {
>  			cgroup_rstat_flush(cgrp);
> @@ -5433,6 +5459,14 @@ static void css_release_work_fn(struct work_struct *work)
>  		cgroup_idr_replace(&ss->css_idr, NULL, css->id);
>  		if (ss->css_released)
>  			ss->css_released(css);
> +
> +		WARN_ON_ONCE(css->nr_descendants || css->nr_dying_descendants);
> +		parent_css = css->parent;
> +		while (parent_css) {
> +			parent_css->nr_dying_descendants--;
> +			parent_css = parent_css->parent;
> +		}
> +		css_put(css->parent);	/* Parent can be freed now */
>  	} else {
>  		struct cgroup *tcgrp;
>  
> @@ -5517,8 +5551,11 @@ static int online_css(struct cgroup_subsys_state *css)
>  		rcu_assign_pointer(css->cgroup->subsys[ss->id], css);
>  
>  		atomic_inc(&css->online_cnt);
> -		if (css->parent)
> +		if (css->parent) {
>  			atomic_inc(&css->parent->online_cnt);
> +			while ((css = css->parent))
> +				css->nr_descendants++;
> +		}
>  	}
>  	return ret;
>  }
> @@ -5540,6 +5577,19 @@ static void offline_css(struct cgroup_subsys_state *css)
>  	RCU_INIT_POINTER(css->cgroup->subsys[ss->id], NULL);
>  
>  	wake_up_all(&css->cgroup->offline_waitq);
> +
> +	/*
> +	 * Get a reference to parent css to ensure reliable access to its
> +	 * nr_dying_descendants until after this child css is ready to be
> +	 * freed.
> +	 */
> +	if (css->parent)
> +		css_get(css->parent);

I think this blob can be dropped: css has a reference to their parent up to
very last moment, see css_free_rwork_fn().
And the corresponding css_put() can be dropped too.

With this thing fixed, please, feel free to add
Acked-by: Roman Gushchin <roman.gushchin@linux.dev> .

Thank you!