cgroup: keep track of number of descent cgroups

Keep track of the number of online and dying descent cgroups.

This data will be used later to add an ability to control cgroup
hierarchy (limit the depth and the number of descent cgroups)
and display hierarchy stats.

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

include/linux/cgroup-defs.h

@@ -273,6 +273,14 @@ struct cgroup {
 	 */
 	int level;
 
+	/*
+	 * Keep track of total numbers of visible and dying descent cgroups.
+	 * Dying cgroups are cgroups which were deleted by a user,
+	 * but are still existing because someone else is holding a reference.
+	 */
+	int nr_descendants;
+	int nr_dying_descendants;
+
 	/*
 	 * Each non-empty css_set associated with this cgroup contributes
 	 * one to nr_populated_csets.  The counter is zero iff this cgroup

kernel/cgroup/cgroup.c

@@ -4408,9 +4408,15 @@ static void css_release_work_fn(struct work_struct *work)
 		if (ss->css_released)
 			ss->css_released(css);
 	} else {
+		struct cgroup *tcgrp;
+
 		/* cgroup release path */
 		trace_cgroup_release(cgrp);
 
+		for (tcgrp = cgroup_parent(cgrp); tcgrp;
+		     tcgrp = cgroup_parent(tcgrp))
+			tcgrp->nr_dying_descendants--;
+
 		cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
 		cgrp->id = -1;
 
@@ -4609,9 +4615,13 @@ static struct cgroup *cgroup_create(struct cgroup *parent)
 	cgrp->root = root;
 	cgrp->level = level;
 
-	for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp))
+	for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp)) {
 		cgrp->ancestor_ids[tcgrp->level] = tcgrp->id;
 
+		if (tcgrp != cgrp)
+			tcgrp->nr_descendants++;
+	}
+
 	if (notify_on_release(parent))
 		set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
 
@@ -4817,7 +4827,7 @@ static void kill_css(struct cgroup_subsys_state *css)
 static int cgroup_destroy_locked(struct cgroup *cgrp)
 	__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
 {
-	struct cgroup *parent = cgroup_parent(cgrp);
+	struct cgroup *tcgrp, *parent = cgroup_parent(cgrp);
 	struct cgroup_subsys_state *css;
 	struct cgrp_cset_link *link;
 	int ssid;
@@ -4865,6 +4875,11 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	if (parent && cgroup_is_threaded(cgrp))
 		parent->nr_threaded_children--;
 
+	for (tcgrp = cgroup_parent(cgrp); tcgrp; tcgrp = cgroup_parent(tcgrp)) {
+		tcgrp->nr_descendants--;
+		tcgrp->nr_dying_descendants++;
+	}
+
 	cgroup1_check_for_release(cgroup_parent(cgrp));
 
 	/* put the base reference */