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

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Date:	Tue, 25 Sep 2012 12:52:53 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	<linux-kernel@...r.kernel.org>
Cc:	<cgroups@...r.kernel.org>, <linux-mm@...ck.org>,
	<devel@...nvz.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Glauber Costa <glommer@...allels.com>,
	Michal Hocko <mhocko@...e.cz>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [RFC 4/4] memcg: do not walk all the way to the root for memcg

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@...allels.com>
CC: Michal Hocko <mhocko@...e.cz>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC: Johannes Weiner <hannes@...xchg.org>
CC: Mel Gorman <mgorman@...e.de>
CC: Andrew Morton <akpm@...ux-foundation.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

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