Commit 7d121d70 authored by Breno Leitao's avatar Breno Leitao Committed by Jens Axboe
Browse files

blk-rq-qos: Remove unlikely() hints from QoS checks 



The unlikely() annotations on QUEUE_FLAG_QOS_ENABLED checks are
counterproductive. Writeback throttling (WBT) might be enabled by
default, mainly because CONFIG_BLK_WBT_MQ defaults to 'y'.

Branch profiling on Meta servers, which have WBT enabled, confirms 100%
misprediction rates on these checks.

Remove the unlikely() annotations to let the CPU's branch predictor
learn the actual behavior, potentially improving I/O path performance.

Signed-off-by: default avatarBreno Leitao <leitao@debian.org>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 08e136eb

block/blk-rq-qos.h

+9 −16
Original line number Diff line number Diff line
@@ -112,29 +112,26 @@ void __rq_qos_queue_depth_changed(struct rq_qos *rqos); 


static inline void rq_qos_cleanup(struct request_queue *q, struct bio *bio)

{

	if (unlikely(test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags)) &&

			q->rq_qos)

	if (test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags) && q->rq_qos)

		__rq_qos_cleanup(q->rq_qos, bio);

}



static inline void rq_qos_done(struct request_queue *q, struct request *rq)

{

	if (unlikely(test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags)) &&

	if (test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags) &&

	    q->rq_qos && !blk_rq_is_passthrough(rq))

		__rq_qos_done(q->rq_qos, rq);

}



static inline void rq_qos_issue(struct request_queue *q, struct request *rq)

{

	if (unlikely(test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags)) &&

			q->rq_qos)

	if (test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags) && q->rq_qos)

		__rq_qos_issue(q->rq_qos, rq);

}



static inline void rq_qos_requeue(struct request_queue *q, struct request *rq)

{

	if (unlikely(test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags)) &&

			q->rq_qos)

	if (test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags) && q->rq_qos)

		__rq_qos_requeue(q->rq_qos, rq);

}
@@ -162,8 +159,7 @@ static inline void rq_qos_done_bio(struct bio *bio) 


static inline void rq_qos_throttle(struct request_queue *q, struct bio *bio)

{

	if (unlikely(test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags)) &&

			q->rq_qos) {

	if (test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags) && q->rq_qos) {

		bio_set_flag(bio, BIO_QOS_THROTTLED);

		__rq_qos_throttle(q->rq_qos, bio);

	}
@@ -172,16 +168,14 @@ static inline void rq_qos_throttle(struct request_queue *q, struct bio *bio) 
static inline void rq_qos_track(struct request_queue *q, struct request *rq,

				struct bio *bio)

{

	if (unlikely(test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags)) &&

			q->rq_qos)

	if (test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags) && q->rq_qos)

		__rq_qos_track(q->rq_qos, rq, bio);

}



static inline void rq_qos_merge(struct request_queue *q, struct request *rq,

				struct bio *bio)

{

	if (unlikely(test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags)) &&

			q->rq_qos) {

	if (test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags) && q->rq_qos) {

		bio_set_flag(bio, BIO_QOS_MERGED);

		__rq_qos_merge(q->rq_qos, rq, bio);

	}
@@ -189,8 +183,7 @@ static inline void rq_qos_merge(struct request_queue *q, struct request *rq, 


static inline void rq_qos_queue_depth_changed(struct request_queue *q)

{

	if (unlikely(test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags)) &&

			q->rq_qos)

	if (test_bit(QUEUE_FLAG_QOS_ENABLED, &q->queue_flags) && q->rq_qos)

		__rq_qos_queue_depth_changed(q->rq_qos);

}