cgroup: add cgroup.stat interface with basic hierarchy stats

A cgroup can consume resources even after being deleted by a user.
For example, writing back dirty pages should be accounted and
limited, despite the corresponding cgroup might contain no processes
and being deleted by a user.

In the current implementation a cgroup can remain in such "dying" state
for an undefined amount of time. For instance, if a memory cgroup
contains a pge, mlocked by a process belonging to an other cgroup.

Although the lifecycle of a dying cgroup is out of user's control,
it's important to have some insight of what's going on under the hood.

In particular, it's handy to have a counter which will allow
to detect css leaks.

To solve this problem, add a cgroup.stat interface to
the base cgroup control files with the following metrics:

nr_descendants		total number of visible descendant cgroups
nr_dying_descendants	total number of dying descendant cgroups

Signed-off-by: Roman Gushchin <guro@fb.com>
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: kernel-team@fb.com
Cc: cgroups@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

Documentation/cgroup-v2.txt

@@ -868,6 +868,24 @@ All cgroup core files are prefixed with "cgroup."
 	If the actual descent depth is equal or larger,
 	an attempt to create a new child cgroup will fail.
 
+  cgroup.stat
+	A read-only flat-keyed file with the following entries:
+
+	  nr_descendants
+		Total number of visible descendant cgroups.
+
+	  nr_dying_descendants
+		Total number of dying descendant cgroups. A cgroup becomes
+		dying after being deleted by a user. The cgroup will remain
+		in dying state for some time undefined time (which can depend
+		on system load) before being completely destroyed.
+
+		A process can't enter a dying cgroup under any circumstances,
+		a dying cgroup can't revive.
+
+		A dying cgroup can consume system resources not exceeding
+		limits, which were active at the moment of cgroup deletion.
+
 
 Controllers
 ===========

kernel/cgroup/cgroup.c

@@ -3304,6 +3304,18 @@ static int cgroup_events_show(struct seq_file *seq, void *v)
 	return 0;
 }
 
+static int cgroup_stats_show(struct seq_file *seq, void *v)
+{
+	struct cgroup *cgroup = seq_css(seq)->cgroup;
+
+	seq_printf(seq, "nr_descendants %d\n",
+		   cgroup->nr_descendants);
+	seq_printf(seq, "nr_dying_descendants %d\n",
+		   cgroup->nr_dying_descendants);
+
+	return 0;
+}
+
 static int cgroup_file_open(struct kernfs_open_file *of)
 {
 	struct cftype *cft = of->kn->priv;
@@ -4407,6 +4419,10 @@ static struct cftype cgroup_base_files[] = {
 		.seq_show = cgroup_max_depth_show,
 		.write = cgroup_max_depth_write,
 	},
+	{
+		.name = "cgroup.stat",
+		.seq_show = cgroup_stats_show,
+	},
 	{ }	/* terminate */
 };