linux-kernel - [PATCH v2 2/2] mm, thp: replace smp_mb after atomic_add by smp_mb__after

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Date:	Tue, 17 Jun 2014 18:37:59 -0400
From:	Waiman Long <Waiman.Long@...com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Scott J Norton <scott.norton@...com>,
	Waiman Long <Waiman.Long@...com>
Subject: [PATCH v2 2/2] mm, thp: replace smp_mb after atomic_add by smp_mb__after_atomic

In some architectures like x86, atomic_add() is a full memory
barrier. In that case, an additional smp_mb() is just a waste of time.
This patch replaces that smp_mb() by smp_mb__after_atomic() which
will avoid the redundant memory barrier in some architectures.

With a 3.16-rc1 based kernel, this patch reduced the execution time
of breaking 1000 transparent huge pages from 38,245us to 30,964us. A
reduction of 19% which is quite sizeable. It also reduces the %cpu
time of the __split_huge_page_refcount function in the perf profile
from 2.18% to 1.15%.

Signed-off-by: Waiman Long <Waiman.Long@...com>
---
 mm/huge_memory.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index be84c71..e2ee131 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1650,7 +1650,7 @@ static void __split_huge_page_refcount(struct page *page,
 			   &page_tail->_count);
 
 		/* after clearing PageTail the gup refcount can be released */
-		smp_mb();
+		smp_mb__after_atomic();
 
 		/*
 		 * retain hwpoison flag of the poisoned tail page:
-- 
1.7.1

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