[PATCH v2 2/2] mm, thp: replace smp_mb after atomic_add by smp_mb__after_atomic

[Waiman Long <Waiman.Long@hp.com>]

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>
---
 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