memcg: hierarchy avoid unnecessary reclaim

If hierarchy is not used, no tree-walk is necessary.
Reviewed-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NDaisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: NKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -606,6 +606,8 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
 	ret = try_to_free_mem_cgroup_pages(root_mem, gfp_mask, noswap);
 	if (mem_cgroup_check_under_limit(root_mem))
 		return 0;
+	if (!root_mem->use_hierarchy)
+		return ret;
 
 	next_mem = mem_cgroup_get_first_node(root_mem);