linux-kernel - Re: [PATCH v2 1/2] blk-mq: fix elevator depth

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Message-ID: <f0abcc02-48c6-4d09-86d6-5ca8b968dcde@linux.ibm.com>
Date: Tue, 19 Aug 2025 17:50:32 +0530
From: Nilay Shroff <nilay@...ux.ibm.com>
To: Yu Kuai <yukuai1@...weicloud.com>, yukuai3@...wei.com, axboe@...nel.dk,
        bvanassche@....org, ming.lei@...hat.com, hare@...e.de
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, yangerkun@...wei.com, johnny.chenyi@...wei.com
Subject: Re: [PATCH v2 1/2] blk-mq: fix elevator depth_updated method



On 8/19/25 6:59 AM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> Current depth_updated has some problems:
> 
> 1) depth_updated() will be called for each hctx, while all elevators
> will update async_depth for the disk level, this is not related to hctx;
> 2) In blk_mq_update_nr_requests(), if previous hctx update succeed and
> this hctx update failed, q->nr_requests will not be updated, while
> async_depth is already updated with new nr_reqeuests in previous
> depth_updated();
> 3) All elevators are using q->nr_requests to calculate async_depth now,
> however, q->nr_requests is still the old value when depth_updated() is
> called from blk_mq_update_nr_requests();
> 
> Fix those problems by:
> 
> - pass in request_queue instead of hctx;
> - move depth_updated() after q->nr_requests is updated in
>   blk_mq_update_nr_requests();
> - add depth_updated() call in blk_mq_init_sched();
> - remove init_hctx() method for mq-deadline and bfq that is useless now;
> 
> Fixes: 77f1e0a52d26 ("bfq: update internal depth state when queue depth changes")
> Fixes: 39823b47bbd4 ("block/mq-deadline: Fix the tag reservation code")
> Fixes: 42e6c6ce03fd ("lib/sbitmap: convert shallow_depth from one word to the whole sbitmap")
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  block/bfq-iosched.c   | 21 ++++-----------------
>  block/blk-mq-sched.c  |  3 +++
>  block/blk-mq-sched.h  | 11 +++++++++++
>  block/blk-mq.c        | 23 ++++++++++++-----------
>  block/elevator.h      |  2 +-
>  block/kyber-iosched.c | 10 ++++------
>  block/mq-deadline.c   | 15 ++-------------
>  7 files changed, 37 insertions(+), 48 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 50e51047e1fe..c0c398998aa1 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -7109,9 +7109,10 @@ void bfq_put_async_queues(struct bfq_data *bfqd, struct bfq_group *bfqg)
>   * See the comments on bfq_limit_depth for the purpose of
>   * the depths set in the function. Return minimum shallow depth we'll use.
>   */
> -static void bfq_update_depths(struct bfq_data *bfqd, struct sbitmap_queue *bt)
> +static void bfq_depth_updated(struct request_queue *q)
>  {
> -	unsigned int nr_requests = bfqd->queue->nr_requests;
> +	struct bfq_data *bfqd = q->elevator->elevator_data;
> +	unsigned int nr_requests = q->nr_requests;
>  
>  	/*
>  	 * In-word depths if no bfq_queue is being weight-raised:
> @@ -7143,21 +7144,8 @@ static void bfq_update_depths(struct bfq_data *bfqd, struct sbitmap_queue *bt)
>  	bfqd->async_depths[1][0] = max((nr_requests * 3) >> 4, 1U);
>  	/* no more than ~37% of tags for sync writes (~20% extra tags) */
>  	bfqd->async_depths[1][1] = max((nr_requests * 6) >> 4, 1U);
> -}
> -
> -static void bfq_depth_updated(struct blk_mq_hw_ctx *hctx)
> -{
> -	struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
> -	struct blk_mq_tags *tags = hctx->sched_tags;
>  
> -	bfq_update_depths(bfqd, &tags->bitmap_tags);
> -	sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, 1);
> -}
> -
> -static int bfq_init_hctx(struct blk_mq_hw_ctx *hctx, unsigned int index)
> -{
> -	bfq_depth_updated(hctx);
> -	return 0;
> +	blk_mq_set_min_shallow_depth(q, 1);
>  }
>  
>  static void bfq_exit_queue(struct elevator_queue *e)
> @@ -7628,7 +7616,6 @@ static struct elevator_type iosched_bfq_mq = {
>  		.request_merged		= bfq_request_merged,
>  		.has_work		= bfq_has_work,
>  		.depth_updated		= bfq_depth_updated,
> -		.init_hctx		= bfq_init_hctx,
>  		.init_sched		= bfq_init_queue,
>  		.exit_sched		= bfq_exit_queue,
>  	},
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index e2ce4a28e6c9..bf7dd97422ec 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -585,6 +585,9 @@ int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e,
>  			}
>  		}
>  	}
> +
> +	if (e->ops.depth_updated)
> +		e->ops.depth_updated(q);
>  	return 0;
>  

Overall changes look good. That said, I think it might be cleaner to structure
it this way:

elevator_switch -> blk_mq_init_sched ->init_sched ==> sets async_depth
blk_mq_update_nr_requests ->depth_updated ==> updates async_depth

This way, we don’t need to call ->depth_updated from blk_mq_init_sched.

In summary:
- Avoid calling ->depth_updated during blk_mq_init_sched
- Set async_depth when the elevator is initialized (via ->init_sched)
- Update async_depth when nr_requests is modified through sysfs (via ->depth_updated)

Thanks,
--Nilay