[PATCH v2 5/5] memcg: do not walk all the way to the root for memcg

[Glauber Costa <glommer@parallels.com>]

Since the root is special anyway, and we always get its figures from
global counters anyway, there is no make all cgroups its descendants,
wrt res_counters. The sad effect of doing that is that we need to lock
the root for all allocations, since it is a common ancestor of
everybody.

Not having the root as a common ancestor should lead to better
scalability for not-uncommon case of tasks in the cgroup being
node-bound to different nodes in NUMA systems.

Signed-off-by: Glauber Costa <glommer@parallels.com>
CC: Michal Hocko <mhocko@suse.cz>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
CC: Johannes Weiner <hannes@cmpxchg.org>
CC: Mel Gorman <mgorman@suse.de>
CC: Andrew Morton <akpm@linux-foundation.org>
---
 mm/memcontrol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6019a32..252dc00 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6464,7 +6464,7 @@ mem_cgroup_css_online(struct cgroup *cont)
 	memcg->oom_kill_disable = parent->oom_kill_disable;
 	memcg->swappiness = mem_cgroup_swappiness(parent);
 
-	if (parent->use_hierarchy) {
+	if (parent && !mem_cgroup_is_root(parent) && parent->use_hierarchy) {
 		res_counter_init(&memcg->res, &parent->res);
 		res_counter_init(&memcg->memsw, &parent->memsw);
 		res_counter_init(&memcg->kmem, &parent->kmem);
-- 
1.8.1.2

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