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: Uros Bizjak <ubizjak@xxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
---
v2: Split patch from the original big patch
Use while instead of do-while loop
---
block/blk-cgroup.c | 2 +-
block/blk-cgroup.h | 12 ++++--------
2 files changed, 5 insertions(+), 9 deletions(-)
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 764e740b0c0f..ca99260348ff 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1696,7 +1696,7 @@ 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);
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index d4de0a35e066..d2724d1dd7c9 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -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);
}
--
2.35.3
