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>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 2161490526f0ebd46f9c4f8e84bdefa1dfc78853..4b95ff4120f5ae49cf01a29b2b921c6501f4c002 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1681,7 +1681,7 @@ 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: