Commit 96388f57 authored by Uros Bizjak's avatar Uros Bizjak Committed by Jens Axboe
Browse files

blk-cgroup: Use atomic{,64}_try_cmpxchg 



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

Also, atomic_try_cmpxchg implicitly assigns old *ptr value to "old" when
cmpxchg fails, enabling further code simplifications.

No functional change intended.

Signed-off-by: default avatarUros Bizjak <ubizjak@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20220712154455.66868-1-ubizjak@gmail.com


Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent aee8960c

block/blk-cgroup.c

+1 −1
Original line number Diff line number Diff line
@@ -1700,7 +1700,7 @@ static void blkcg_scale_delay(struct blkcg_gq *blkg, u64 now) 
	 * everybody is happy with their IO latencies.

	 */

	if (time_before64(old + NSEC_PER_SEC, now) &&

	    atomic64_cmpxchg(&blkg->delay_start, old, now) == old) {

	    atomic64_try_cmpxchg(&blkg->delay_start, &old, now)) {

		u64 cur = atomic64_read(&blkg->delay_nsec);

		u64 sub = min_t(u64, blkg->last_delay, now - old);

		int cur_use = atomic_read(&blkg->use_delay);

block/blk-cgroup.h

+4 −8
Original line number Diff line number Diff line
@@ -430,12 +430,8 @@ static inline int blkcg_unuse_delay(struct blkcg_gq *blkg) 
	 * then check to see if we were the last delay so we can drop the

	 * congestion count on the cgroup.

	 */

	while (old) {

		int cur = atomic_cmpxchg(&blkg->use_delay, old, old - 1);

		if (cur == old)

			break;

		old = cur;

	}

	while (old && !atomic_try_cmpxchg(&blkg->use_delay, &old, old - 1))

		;



	if (old == 0)

		return 0;
@@ -458,7 +454,7 @@ static inline void blkcg_set_delay(struct blkcg_gq *blkg, u64 delay) 
	int old = atomic_read(&blkg->use_delay);



	/* We only want 1 person setting the congestion count for this blkg. */

	if (!old && atomic_cmpxchg(&blkg->use_delay, old, -1) == old)

	if (!old && atomic_try_cmpxchg(&blkg->use_delay, &old, -1))

		atomic_inc(&blkg->blkcg->css.cgroup->congestion_count);



	atomic64_set(&blkg->delay_nsec, delay);
@@ -475,7 +471,7 @@ static inline void blkcg_clear_delay(struct blkcg_gq *blkg) 
	int old = atomic_read(&blkg->use_delay);



	/* We only want 1 person clearing the congestion count for this blkg. */

	if (old && atomic_cmpxchg(&blkg->use_delay, old, 0) == old)

	if (old && atomic_try_cmpxchg(&blkg->use_delay, &old, 0))

		atomic_dec(&blkg->blkcg->css.cgroup->congestion_count);

}