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: Uros 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: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 27a2d0c..869af9d 100644 (file)
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -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);

diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index d4de0a3..d2724d1 100644 (file)
--- 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);
 }