linux-kernel - [PATCH v3 2/8] mm, compaction: remove redundant watermark check in compact

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Date:   Tue,  7 Mar 2017 14:15:39 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>, kernel-team@...com,
        Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH v3 2/8] mm, compaction: remove redundant watermark check in compact_finished()

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@...e.cz>
Acked-by: Mel Gorman <mgorman@...hsingularity.net>
Acked-by: Johannes Weiner <hannes@...xchg.org>
---
 mm/compaction.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 81e1eaa2a2cf..9222ff362f33 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1280,7 +1280,6 @@ 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;
@@ -1308,13 +1307,6 @@ 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];
-- 
2.12.0