revert "memcg: track all children over limit in the root"

Revert commit 1be171d ("memcg: track all children over limit in the
root")

I merged this prematurely - Michal and Johannes still disagree about the
overall design direction and the future remains unclear.

Cc: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -851,15 +851,9 @@ static void mem_cgroup_update_soft_limit(struct mem_cgroup *memcg)
 	/*
 	 * Necessary to update all ancestors when hierarchy is used
 	 * because their event counter is not touched.
-	 * We track children even outside the hierarchy for the root
-	 * cgroup because tree walk starting at root should visit
-	 * all cgroups and we want to prevent from pointless tree
-	 * walk if no children is below the limit.
 	 */
 	while (delta && (parent = parent_mem_cgroup(parent)))
 		atomic_add(delta, &parent->children_in_excess);
-	if (memcg != root_mem_cgroup && !root_mem_cgroup->use_hierarchy)
-		atomic_add(delta, &root_mem_cgroup->children_in_excess);
 	spin_unlock(&memcg->soft_lock);
 }
 
@@ -6112,9 +6106,6 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
 	if (memcg->soft_contributed) {
 		while ((memcg = parent_mem_cgroup(memcg)))
 			atomic_dec(&memcg->children_in_excess);
-
-		if (memcg != root_mem_cgroup && !root_mem_cgroup->use_hierarchy)
-			atomic_dec(&root_mem_cgroup->children_in_excess);
 	}
 	mem_cgroup_destroy_all_caches(memcg);
 	vmpressure_cleanup(&memcg->vmpressure);