Re: [PATCH] block: blk-rq-qos: guard rq-qos helpers by static key

[Yu Kuai <yukuai1@xxxxxxxxxxxxxxx>]

在 2025/04/15 22:52, Jens Axboe 写道:
> Even if blk-rq-qos isn't used or configured, dipping into the queue to
> fetch ->rq_qos is a noticeable slowdown and visible in profiles. Add an
> unlikely static key around blk-rq-qos, to avoid fetching this cacheline
> if blk-iolatency or blk-wbt isn't configured or used.
>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
>
> ---
LGTM

Reviewed-by: Yu Kuai <yukuai3@xxxxxxxxxx>

BTW, do we want the same thing for q->td?

Thanks,
Kuai

> diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
> index 95982bc46ba1..848591fb3c57 100644
> --- a/block/blk-rq-qos.c
> +++ b/block/blk-rq-qos.c
> @@ -2,6 +2,8 @@
>   
>   #include "blk-rq-qos.h"
>   
> +__read_mostly DEFINE_STATIC_KEY_FALSE(block_rq_qos);
> +
>   /*
>    * Increment 'v', if 'v' is below 'below'. Returns true if we succeeded,
>    * false if 'v' + 1 would be bigger than 'below'.
> @@ -317,6 +319,7 @@ void rq_qos_exit(struct request_queue *q)
>   		struct rq_qos *rqos = q->rq_qos;
>   		q->rq_qos = rqos->next;
>   		rqos->ops->exit(rqos);
> +		static_branch_dec(&block_rq_qos);
>   	}
>   	mutex_unlock(&q->rq_qos_mutex);
>   }
> @@ -343,6 +346,7 @@ int rq_qos_add(struct rq_qos *rqos, struct gendisk *disk, enum rq_qos_id id,
>   		goto ebusy;
>   	rqos->next = q->rq_qos;
>   	q->rq_qos = rqos;
> +	static_branch_inc(&block_rq_qos);
>   
>   	blk_mq_unfreeze_queue(q, memflags);
>   
> diff --git a/block/blk-rq-qos.h b/block/blk-rq-qos.h
> index 37245c97ee61..39749f4066fb 100644
> --- a/block/blk-rq-qos.h
> +++ b/block/blk-rq-qos.h
> @@ -12,6 +12,7 @@
>   #include "blk-mq-debugfs.h"
>   
>   struct blk_mq_debugfs_attr;
> +extern struct static_key_false block_rq_qos;
>   
>   enum rq_qos_id {
>   	RQ_QOS_WBT,
> @@ -112,31 +113,33 @@ void __rq_qos_queue_depth_changed(struct rq_qos *rqos);
>   
>   static inline void rq_qos_cleanup(struct request_queue *q, struct bio *bio)
>   {
> -	if (q->rq_qos)
> +	if (static_branch_unlikely(&block_rq_qos) && q->rq_qos)
>   		__rq_qos_cleanup(q->rq_qos, bio);
>   }
>   
>   static inline void rq_qos_done(struct request_queue *q, struct request *rq)
>   {
> -	if (q->rq_qos && !blk_rq_is_passthrough(rq))
> +	if (static_branch_unlikely(&block_rq_qos) && 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 (q->rq_qos)
> +	if (static_branch_unlikely(&block_rq_qos) && q->rq_qos)
>   		__rq_qos_issue(q->rq_qos, rq);
>   }
>   
>   static inline void rq_qos_requeue(struct request_queue *q, struct request *rq)
>   {
> -	if (q->rq_qos)
> +	if (static_branch_unlikely(&block_rq_qos) && q->rq_qos)
>   		__rq_qos_requeue(q->rq_qos, rq);
>   }
>   
>   static inline void rq_qos_done_bio(struct bio *bio)
>   {
> -	if (bio->bi_bdev && (bio_flagged(bio, BIO_QOS_THROTTLED) ||
> +	if (static_branch_unlikely(&block_rq_qos) &&
> +	    bio->bi_bdev && (bio_flagged(bio, BIO_QOS_THROTTLED) ||
>   			     bio_flagged(bio, BIO_QOS_MERGED))) {
>   		struct request_queue *q = bdev_get_queue(bio->bi_bdev);
>   		if (q->rq_qos)
> @@ -146,7 +149,7 @@ static inline void rq_qos_done_bio(struct bio *bio)
>   
>   static inline void rq_qos_throttle(struct request_queue *q, struct bio *bio)
>   {
> -	if (q->rq_qos) {
> +	if (static_branch_unlikely(&block_rq_qos) && q->rq_qos) {
>   		bio_set_flag(bio, BIO_QOS_THROTTLED);
>   		__rq_qos_throttle(q->rq_qos, bio);
>   	}
> @@ -155,14 +158,14 @@ 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 (q->rq_qos)
> +	if (static_branch_unlikely(&block_rq_qos) && 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 (q->rq_qos) {
> +	if (static_branch_unlikely(&block_rq_qos) && q->rq_qos) {
>   		bio_set_flag(bio, BIO_QOS_MERGED);
>   		__rq_qos_merge(q->rq_qos, rq, bio);
>   	}
> @@ -170,7 +173,7 @@ 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 (q->rq_qos)
> +	if (static_branch_unlikely(&block_rq_qos) && q->rq_qos)
>   		__rq_qos_queue_depth_changed(q->rq_qos);
>   }
>