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

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

[Breno Leitao]

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: Breno Leitao <leitao@debian.org>
---
 block/blk-rq-qos.h | 25 +++++++++----------------
 1 file changed, 9 insertions(+), 16 deletions(-)

diff --git a/block/blk-rq-qos.h b/block/blk-rq-qos.h
index b538f2c0febc..a747a504fe42 100644
--- a/block/blk-rq-qos.h
+++ b/block/blk-rq-qos.h
@@ -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)) &&
-			q->rq_qos && !blk_rq_is_passthrough(rq))
+	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);
 }
 

---
base-commit: c92f16e43c48f367af2f63956eb8385cb05816f3
change-id: 20260106-blk_unlikely-6ea4cf76b104

Best regards,
--  
Breno Leitao <leitao@debian.org>

Re: [PATCH] blk-rq-qos: Remove unlikely() hints from QoS checks

[Jens Axboe]

On Tue, 06 Jan 2026 06:26:57 -0800, Breno Leitao wrote:
> 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.
> 
> [...]

Applied, thanks!

[1/1] blk-rq-qos: Remove unlikely() hints from QoS checks
      commit: 7d121d701d58a92f26decb10da1d04a88b74519d

Best regards,
-- 
Jens Axboe