From: Waiman Long <Waiman.Long@hp.com>
Date: Wed, 6 Aug 2014 23:05:38 +0000 (-0700)
Subject: mm, thp: replace smp_mb after atomic_add by smp_mb__after_atomic
X-Git-Tag: cleanup-for-v3.18~96^2~162
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3a79d52aa3c63c939f5a1f86e80e634f84e987c4;p=pandora-kernel.git

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@hp.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Scott J Norton <scott.norton@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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