cgroup/freezer: Reduce redundant traversal for cgroup_freeze

Whether a cgroup is frozen is determined solely by whether it is set to
to be frozen and whether its parent is frozen. Currently, when is cgroup
is frozen or unfrozen, it iterates through the entire subtree to freeze
or unfreeze its descentdants. However, this is unesessary for a cgroup
that does not change its effective frozen status. This path aims to skip
the subtree if its parent does not have a change in effective freeze.

For an example, subtree like, a-b-c-d-e-f-g, when a is frozen, the
entire tree is frozen. If we freeze b and c again, it is unesessary to
iterate d, e, f and g. So does that If we unfreeze b/c.

Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: Chen Ridong

include/linux/cgroup-defs.h

@@ -398,7 +398,7 @@ struct cgroup_freezer_state {
 	bool freeze;
 
 	/* Should the cgroup actually be frozen? */
-	int e_freeze;
+	bool e_freeze;
 
 	/* Fields below are protected by css_set_lock */
 

kernel/cgroup/freezer.c

@@ -260,8 +260,10 @@ void cgroup_freezer_migrate_task(struct task_struct *task,
 void cgroup_freeze(struct cgroup *cgrp, bool freeze)
 {
 	struct cgroup_subsys_state *css;
+	struct cgroup *parent;
 	struct cgroup *dsct;
 	bool applied = false;
+	bool old_e;
 
 	lockdep_assert_held(&cgroup_mutex);
 
@@ -282,22 +284,18 @@ void cgroup_freeze(struct cgroup *cgrp, bool freeze)
 		if (cgroup_is_dead(dsct))
 			continue;
 
-		if (freeze) {
-			dsct->freezer.e_freeze++;
-			/*
-			 * Already frozen because of ancestor's settings?
-			 */
-			if (dsct->freezer.e_freeze > 1)
-				continue;
-		} else {
-			dsct->freezer.e_freeze--;
-			/*
-			 * Still frozen because of ancestor's settings?
-			 */
-			if (dsct->freezer.e_freeze > 0)
-				continue;
-
-			WARN_ON_ONCE(dsct->freezer.e_freeze < 0);
+		/*
+		 * e_freeze is affected by parent's e_freeze and dst's freeze.
+		 * If old e_freeze eq new e_freeze, no change, its children
+		 * will not be affected. So do nothing and skip the subtree
+		 */
+		old_e = dsct->freezer.e_freeze;
+		parent = cgroup_parent(dsct);
+		dsct->freezer.e_freeze = (dsct->freezer.freeze ||
+					  parent->freezer.e_freeze);
+		if (dsct->freezer.e_freeze == old_e) {
+			css = css_rightmost_descendant(css);
+			continue;
 		}
 
 		/*