[PATCH v2 02/10] mm, compaction: remove redundant watermark check in compact_finished()

Vlastimil Babka
2017-02-10T19:13:47Z

When detecting whether compaction has succeeded in forming a high-order page,
__compact_finished() employs a watermark check, followed by an own search for
a suitable page in the freelists. This is not ideal for two reasons:

- The watermark check also searches high-order freelists, but has a less strict
criteria wrt fallback. It's therefore redundant and waste of cycles. This was
different in the past when high-order watermark check attempted to apply
reserves to high-order pages.

- The watermark check might actually fail due to lack of order-0 pages.
Compaction can't help with that, so there's no point in continuing because of
that. It's possible that high-order page still exists and it terminates.

This patch therefore removes the watermark check. This should save some cycles
and terminate compaction sooner in some cases.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/compaction.c | 8 --------
1 file changed, 8 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 0409a4ad6ea1..fc88e7b6fe37 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1291,7 +1291,6 @@ static enum compact_result __compact_finished(struct zone *zone, struct compact_
const int migratetype)
{
unsigned int order;
- unsigned long watermark;

if (cc->contended || fatal_signal_pending(current))
return COMPACT_CONTENDED;
@@ -1319,13 +1318,6 @@ static enum compact_result __compact_finished(struct zone *zone, struct compact_
if (is_via_compact_memory(cc->order))
return COMPACT_CONTINUE;

- /* Compaction run is not finished if the watermark is not met */
- watermark = zone->watermark[cc->alloc_flags & ALLOC_WMARK_MASK];
-
- if (!zone_watermark_ok(zone, cc->order, watermark, cc->classzone_idx,
- cc->alloc_flags))
- return COMPACT_CONTINUE;
-
/* Direct compactor: Is a suitable page free? */
for (order = cc->order; order < MAX_ORDER; order++) {
struct free_area *area = &zone->free_area[order];
--
2.11.0