linux-kernel - [PATCH 18/28] mm, page_alloc: Shorten the page allocator fast path

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Message-Id: <1460711275-1130-6-git-send-email-mgorman@techsingularity.net>
Date:	Fri, 15 Apr 2016 10:07:45 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...hsingularity.net>
Subject: [PATCH 18/28] mm, page_alloc: Shorten the page allocator fast path

The page allocator fast path checks page multiple times unnecessarily.
This patch avoids all the slowpath checks if the first allocation attempt
succeeds.

Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
---
 mm/page_alloc.c | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 835a1c434832..7a5f6ff4ea06 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3392,22 +3392,17 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
 
 	/* First allocation attempt */
 	page = get_page_from_freelist(alloc_mask, order, alloc_flags, &ac);
-	if (unlikely(!page)) {
-		/*
-		 * Runtime PM, block IO and its error handling path
-		 * can deadlock because I/O on the device might not
-		 * complete.
-		 */
-		alloc_mask = memalloc_noio_flags(gfp_mask);
-		ac.spread_dirty_pages = false;
-
-		page = __alloc_pages_slowpath(alloc_mask, order, &ac);
-	}
+	if (likely(page))
+		goto out;
 
-	if (kmemcheck_enabled && page)
-		kmemcheck_pagealloc_alloc(page, order, gfp_mask);
+	/*
+	 * Runtime PM, block IO and its error handling path can deadlock
+	 * because I/O on the device might not complete.
+	 */
+	alloc_mask = memalloc_noio_flags(gfp_mask);
+	ac.spread_dirty_pages = false;
 
-	trace_mm_page_alloc(page, order, alloc_mask, ac.migratetype);
+	page = __alloc_pages_slowpath(alloc_mask, order, &ac);
 
 	/*
 	 * When updating a task's mems_allowed, it is possible to race with
@@ -3420,6 +3415,12 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
 		goto retry_cpuset;
 	}
 
+out:
+	if (kmemcheck_enabled && page)
+		kmemcheck_pagealloc_alloc(page, order, gfp_mask);
+
+	trace_mm_page_alloc(page, order, alloc_mask, ac.migratetype);
+
 	return page;
 }
 EXPORT_SYMBOL(__alloc_pages_nodemask);
-- 
2.6.4