[RFC 4/4] memcg: do not walk all the way to the root for memcg

[Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>]

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-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>
CC: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
CC: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
CC: Mel Gorman <mgorman-l3A5Bk7waGM@public.gmane.org>
CC: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
---
 mm/memcontrol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f8115f0..829ea9e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5720,7 +5720,7 @@ mem_cgroup_create(struct cgroup *cont)
 		static_key_slow_inc(&memcg_in_use_key);
 	}
 
-	if (parent && parent->use_hierarchy) {
+	if (parent && !mem_cgroup_is_root(parent) && parent->use_hierarchy) {
 		struct mem_cgroup __maybe_unused *p;
 
 		res_counter_init(&memcg->res, &parent->res);
-- 
1.7.11.4