linux-kernel - [PATCH] mm: vmscan: consistent update to pgsteal and pgscan

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Message-Id: <20200507204913.18661-1-shakeelb@google.com>
Date:   Thu,  7 May 2020 13:49:13 -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>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Shakeel Butt <shakeelb@...gle.com>
Subject: [PATCH] mm: vmscan: consistent update to pgsteal and pgscan

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 but root does not expose that
interface. Also for !CONFIG_MEMCG machines /proc/vmstat is the only way
to get these stats. So, make these stats consistent.

Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
---
 mm/vmscan.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index cc555903a332..51f7d1efc912 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1943,8 +1943,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 	reclaim_stat->recent_scanned[file] += nr_taken;

 	item = current_is_kswapd() ? PGSCAN_KSWAPD : PGSCAN_DIRECT;
-	if (!cgroup_reclaim(sc))
-		__count_vm_events(item, nr_scanned);
+	__count_vm_events(item, nr_scanned);
 	__count_memcg_events(lruvec_memcg(lruvec), item, nr_scanned);
 	spin_unlock_irq(&pgdat->lru_lock);

@@ -1957,8 +1956,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 	spin_lock_irq(&pgdat->lru_lock);

 	item = current_is_kswapd() ? PGSTEAL_KSWAPD : PGSTEAL_DIRECT;
-	if (!cgroup_reclaim(sc))
-		__count_vm_events(item, nr_reclaimed);
+	__count_vm_events(item, nr_reclaimed);
 	__count_memcg_events(lruvec_memcg(lruvec), item, nr_reclaimed);
 	reclaim_stat->recent_rotated[0] += stat.nr_activate[0];
 	reclaim_stat->recent_rotated[1] += stat.nr_activate[1];
-- 
2.26.2.526.g744177e7f7-goog