mm, thp: replace smp_mb after atomic_add by smp_mb__after_atomic
authorWaiman Long <Waiman.Long@hp.com>
Wed, 6 Aug 2014 23:05:38 +0000 (16:05 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Thu, 7 Aug 2014 01:01:17 +0000 (18:01 -0700)
commit3a79d52aa3c63c939f5a1f86e80e634f84e987c4
treeea352191eb8bc6afbc98ac3b54d573c1db615c78
parentf8303c2582b889351e261ff18c4d8eb197a77db2
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>
mm/huge_memory.c