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buffer: Avoid setting buffer bits that are already set

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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>
Tested-by: Kemi Wang <kemi.wang@intel.com>
Signed-off-by: Kemi Wang <kemi.wang@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Kemi Wang 2017-10-24 09:16:42 +08:00 committed by Jens Axboe
parent bea99a5007
commit 60f91826ca
1 changed files with 4 additions and 1 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

5
include/linux/buffer_head.h
View file

@ -81,11 +81,14 @@ 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) \
{ \