From: Breno Leitao <leitao@debian.org>
To: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
tj@kernel.org, josef@toxicpanda.com, kernel-team@meta.com,
Breno Leitao <leitao@debian.org>
Subject: [PATCH] blk-rq-qos: Remove unlikely() hints from QoS checks
Date: Tue, 06 Jan 2026 06:26:57 -0800 [thread overview]
Message-ID: <20260106-blk_unlikely-v1-1-90fb556a6776@debian.org> (raw)
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>
next reply other threads:[~2026-01-06 14:27 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-01-06 14:26 Breno Leitao [this message]
2026-01-07 2:10 ` Jens Axboe
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20260106-blk_unlikely-v1-1-90fb556a6776@debian.org \
--to=leitao@debian.org \
--cc=axboe@kernel.dk \
--cc=josef@toxicpanda.com \
--cc=kernel-team@meta.com \
--cc=linux-block@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=tj@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox
all inboxes | Powered by JetHome®