[PATCH 3/5] mm, compaction: defer only on COMPACT_COMPLETE

[Vlastimil Babka]

Deferred compaction is employed to avoid compacting zone where sync direct
compaction has recently failed. As such, it makes sense to only defer when
a full zone was scanned, which is when compact_zone returns with
COMPACT_COMPLETE. It's less useful to defer when compact_zone returns with
apparent success (COMPACT_PARTIAL), followed by a watermark check failure,
which can happen due to parallel allocation activity. It also does not make
much sense to defer compaction which was completely skipped (COMPACT_SKIP) for
being unsuitable in the first place.

This patch therefore makes deferred compaction trigger only when
COMPACT_COMPLETE is returned from compact_zone(). Results of stress-highalloc
becnmark show the difference is within measurement error, so the issue is
rather cosmetic.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
---
 mm/compaction.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 4b3e0bd..9107588 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1340,7 +1340,7 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
 			goto break_loop;
 		}
 
-		if (mode != MIGRATE_ASYNC) {
+		if (mode != MIGRATE_ASYNC && status == COMPACT_COMPLETE) {
 			/*
 			 * We think that allocation won't succeed in this zone
 			 * so we defer compaction there. If it ends up
-- 
1.8.4.5

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