[PATCH] memcg,vmscan: do not break out targeted reclaim withoutreclaimed pages

[Michal Hocko]

Targeted (hard resp. soft) reclaim has traditionally tried to scan one
group with decreasing priority until nr_to_reclaim (SWAP_CLUSTER_MAX
pages) is reclaimed or all priorities are exhausted. The reclaim is
then retried until the limit is met.

This approach, however, doesn't work well with deeper hierarchies where
groups higher in the hierarchy do not have any or only very few pages
(this usually happens if those groups do not have any tasks and they
have only re-parented pages after some of their children is removed).
Those groups are reclaimed with decreasing priority pointlessly as there
is nothing to reclaim from them.

An easiest fix is to break out of the memcg iteration loop in shrink_zone
only if the whole hierarchy has been visited or sufficient pages have
been reclaimed. This is also more natural because the reclaimer expects
that the hierarchy under the given root is reclaimed. As a result we can
simplify the soft limit reclaim which does its own iteration.

Reported-by: Ying Han <yinghan@xxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 mm/vmscan.c |   16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 53dcde9..161e3ad 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1912,16 +1912,16 @@ static void shrink_zone(struct zone *zone, struct scan_control *sc)
 		shrink_lruvec(lruvec, sc);
 
 		/*
-		 * Limit reclaim has historically picked one memcg and
-		 * scanned it with decreasing priority levels until
-		 * nr_to_reclaim had been reclaimed.  This priority
-		 * cycle is thus over after a single memcg.
+		 * Direct reclaim and kswapd have to scan all memory cgroups
+		 * to fulfill the overall scan target for the zone.
 		 *
-		 * Direct reclaim and kswapd, on the other hand, have
-		 * to scan all memory cgroups to fulfill the overall
-		 * scan target for the zone.
+		 * Limit reclaim, on the other hand, only cares about
+		 * nr_to_reclaim pages to be reclaimed and it will retry with
+		 * decreasing priority if one round over the whole hierarchy
+		 * is not sufficient.
 		 */
-		if (!global_reclaim(sc)) {
+		if (!global_reclaim(sc) &&
+				sc->nr_to_reclaim >= sc->nr_reclaimed) {
 			mem_cgroup_iter_break(root, memcg);
 			break;
 		}
-- 
1.7.10.4

-- 
Michal Hocko
SUSE Labs
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