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From:   Kemi Wang <kemi.wang@intel.com>
To:     Jan Kara <jack@suse.cz>, Jens Axboe <axboe@fb.com>,
        Darrick J Wong <darrick.wong@oracle.com>,
        Kemi Wang <kemi.wang@intel.com>,
        Eric Biggers <ebiggers@google.com>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Jeff Layton <jlayton@redhat.com>
Cc:     Dave <dave.hansen@linux.intel.com>,
        Andi Kleen <andi.kleen@intel.com>,
        Tim Chen <tim.c.chen@intel.com>,
        Ying Huang <ying.huang@intel.com>,
        Aaron Lu <aaron.lu@intel.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>
Subject: [PATCH v2] buffer: Avoid setting buffer bits that are already set
Date:   Tue, 24 Oct 2017 09:16:42 +0800
Message-Id: <1508807802-9691-1-git-send-email-kemi.wang@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@linux.intel.com>
Signed-off-by: Kemi Wang <kemi.wang@intel.com>
Tested-by: Kemi Wang <kemi.wang@intel.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/buffer_head.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index c8dae55..211d8f5 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -80,11 +80,14 @@ struct buffer_head {
 /*
  * macro tricks to expand the set_buffer_foo(), clear_buffer_foo()
  * and buffer_foo() functions.
+ * To avoid reset buffer flags that are already set, because that causes
+ * a costly cache line transition, check the flag first.
  */
 #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


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