[PATCH] buffer: Avoid setting buffer bits that are already set

[Kemi Wang]

It's expensive to set buffer flags that are already set, because that
causes a costly cache line transition.

A common case is setting the "verified" flag during ext4 writes.
This patch checks for the flag being set first.

With the AIM7/creat-clo benchmark testing on a 48G ramdisk based-on ext4
file system, we see 3.3%(15431->15936) improvement of aim7.jobs-per-min on
a 2-sockets broadwell platform.

What the benchmark does is: it forks 3000 processes, and each  process do
the following:
a) open a new file
b) close the file
c) delete the file
until loop=100*1000 times.

The original patch is contributed by Andi Kleen.

Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Kemi Wang <kemi.wang@xxxxxxxxx>
Tested-by: Kemi Wang <kemi.wang@xxxxxxxxx>
---
 include/linux/buffer_head.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index c8dae55..e1799f7 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -84,7 +84,8 @@ struct buffer_head {
 #define BUFFER_FNS(bit, name)						\
 static __always_inline void set_buffer_##name(struct buffer_head *bh)	\
 {									\
-	set_bit(BH_##bit, &(bh)->b_state);				\
+	if (!test_bit(BH_##bit, &(bh)->b_state))			\
+		set_bit(BH_##bit, &(bh)->b_state);              \
 }									\
 static __always_inline void clear_buffer_##name(struct buffer_head *bh)	\
 {									\
-- 
2.7.4