2022-07-12 15:29:48

by Uros Bizjak

Subject: [PATCH v2] blk-iolatency: Use atomic{,64}_try_cmpxchg

Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
in check_scale_change and atomic64_try_cmpxchg in blkcg_iolatency_done_bio.
x86 CMPXCHG instruction returns success in ZF flag, so this change saves a
compare after cmpxchg (and related move instruction in front of cmpxchg).

No functional change intended.

Signed-off-by: Uros Bizjak <[email protected]>
Cc: Jens Axboe <[email protected]>
---
v2: Split patch from the original big patch
---
block/blk-iolatency.c | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index 9568bf8dfe82..79745c6d8e15 100644
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -401,7 +401,6 @@ static void check_scale_change(struct iolatency_grp *iolat)
unsigned int cur_cookie;
unsigned int our_cookie = atomic_read(&iolat->scale_cookie);
u64 scale_lat;
- unsigned int old;
int direction = 0;

if (lat_to_blkg(iolat)->parent == NULL)
@@ -422,11 +421,10 @@ static void check_scale_change(struct iolatency_grp *iolat)
else
return;

- old = atomic_cmpxchg(&iolat->scale_cookie, our_cookie, cur_cookie);
-
- /* Somebody beat us to the punch, just bail. */
- if (old != our_cookie)
+ if (!atomic_try_cmpxchg(&iolat->scale_cookie, &our_cookie, cur_cookie)) {
+ /* Somebody beat us to the punch, just bail. */
return;
+ }

if (direction < 0 && iolat->min_lat_nsec) {
u64 samples_thresh;
@@ -633,8 +631,8 @@ static void blkcg_iolatency_done_bio(struct rq_qos *rqos, struct bio *bio)
window_start = atomic64_read(&iolat->window_start);
if (now > window_start &&
(now - window_start) >= iolat->cur_win_nsec) {
- if (atomic64_cmpxchg(&iolat->window_start,
- window_start, now) == window_start)
+ if (atomic64_try_cmpxchg(&iolat->window_start,
+ &window_start, now))
iolatency_check_latencies(iolat, now);
}
}
--
2.35.3

2022-07-12 21:57:36

by Jens Axboe

Subject: Re: [PATCH v2] blk-iolatency: Use atomic{,64}_try_cmpxchg

On Tue, 12 Jul 2022 17:19:47 +0200, Uros Bizjak wrote:
> Use atomic_try_cmpxchg instead of atomic_cmpxchg (*ptr, old, new) == old
> in check_scale_change and atomic64_try_cmpxchg in blkcg_iolatency_done_bio.
> x86 CMPXCHG instruction returns success in ZF flag, so this change saves a
> compare after cmpxchg (and related move instruction in front of cmpxchg).
>
> No functional change intended.
>
> [...]

Applied, thanks!

[1/1] blk-iolatency: Use atomic{,64}_try_cmpxchg
commit: aee8960c2eae12636040dbf0f04e135273b1612d

Best regards,
--
Jens Axboe