linux-kernel - [PATCH v2 mmotm 1/3] mm, page_alloc: add unlikely macro to help compiler optimization

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Date:	Tue,  6 Aug 2013 17:37:33 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Joonsoo Kim <js1304@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: [PATCH v2 mmotm 1/3] mm, page_alloc: add unlikely macro to help compiler optimization

We rarely allocate a page with ALLOC_NO_WATERMARKS and it is used
in slow path. For helping compiler optimization, add unlikely macro to
ALLOC_NO_WATERMARKS checking.

This patch doesn't have any effect now, because gcc already optimize
this properly. But we cannot assume that gcc always does right and nobody
re-evaluate if gcc do proper optimization with their change, for example,
it is not optimized properly on v3.10. So adding compiler hint here
is reasonable.

Acked-by: Johannes Weiner <hannes@...xchg.org>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index f5c549c..04bec49 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1901,7 +1901,7 @@ zonelist_scan:
 			!cpuset_zone_allowed_softwall(zone, gfp_mask))
 				continue;
 		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
-		if (alloc_flags & ALLOC_NO_WATERMARKS)
+		if (unlikely(alloc_flags & ALLOC_NO_WATERMARKS))
 			goto try_this_zone;
 		/*
 		 * Distribute pages in proportion to the individual
-- 
1.7.9.5

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