linux-kernel - [PATCH] memcg: expose root cgroup's memory.stat

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Message-Id: <20200508170630.94406-1-shakeelb@google.com>
Date:   Fri,  8 May 2020 10:06:30 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Mel Gorman <mgorman@...e.de>, Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Yafang Shao <laoar.shao@...il.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Shakeel Butt <shakeelb@...gle.com>
Subject: [PATCH] memcg: expose root cgroup's memory.stat

One way to measure the efficiency of memory reclaim is to look at the
ratio (pgscan+pfrefill)/pgsteal. However at the moment these stats are
not updated consistently at the system level and the ratio of these are
not very meaningful. The pgsteal and pgscan are updated for only global
reclaim while pgrefill gets updated for global as well as cgroup
reclaim.

Please note that this difference is only for system level vmstats. The
cgroup stats returned by memory.stat are actually consistent. The
cgroup's pgsteal contains number of reclaimed pages for global as well
as cgroup reclaim. So, one way to get the system level stats is to get
these stats from root's memory.stat, so, expose memory.stat for the root
cgroup.

	from Johannes Weiner:
	There are subtle differences between /proc/vmstat and
	memory.stat, and cgroup-aware code that wants to watch the full
	hierarchy currently has to know about these intricacies and
	translate semantics back and forth.

	Generally having the fully recursive memory.stat at the root
	level could help a broader range of usecases.

Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
Suggested-by: Johannes Weiner <hannes@...xchg.org>
---
 mm/memcontrol.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 05dcb72314b5..c300d52c07a5 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6230,7 +6230,6 @@ static struct cftype memory_files[] = {
 	},
 	{
 		.name = "stat",
-		.flags = CFTYPE_NOT_ON_ROOT,
 		.seq_show = memory_stat_show,
 	},
 	{
-- 
2.26.2.645.ge9eca65c58-goog