linux-kernel - Re: [PATCH v3 1/2] blk-mq: fix elevator depth

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Message-ID: <6aa22a6d-a037-4b4b-b403-1e36838602a1@suse.de>
Date: Fri, 22 Aug 2025 08:00:34 +0200
From: Hannes Reinecke <hare@...e.de>
To: Yu Kuai <yukuai1@...weicloud.com>, yukuai3@...wei.com, axboe@...nel.dk,
 bvanassche@....org, ming.lei@...hat.com, nilay@...ux.ibm.com
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 v3 1/2] blk-mq: fix elevator depth_updated method

On 8/21/25 08:06, 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();
> 
> Those problems are first from error path, then mq-deadline, and recently
> for bfq and kyber, 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 inside init_sched() method to initialize
>    async_depth;
> - 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   | 22 +++++-----------------
>   block/blk-mq-sched.h  | 11 +++++++++++
>   block/blk-mq.c        | 23 ++++++++++++-----------
>   block/elevator.h      |  2 +-
>   block/kyber-iosched.c | 19 +++++++++----------
>   block/mq-deadline.c   | 16 +++-------------
>   6 files changed, 41 insertions(+), 52 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare@...e.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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