linux-kernel - Re: [PATCH 2/2] Revert "mq-deadline: Fix request accounting"

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Date:   Tue, 7 Sep 2021 07:54:09 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Niklas Cassel <Niklas.Cassel@....com>, Jens Axboe <axboe@...nel.dk>
Cc:     Damien Le Moal <Damien.LeMoal@....com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Ming Lei <ming.lei@...hat.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] Revert "mq-deadline: Fix request accounting"

On 9/7/21 7:21 AM, Niklas Cassel wrote:
> From: Niklas Cassel <niklas.cassel@....com>
> 
> This reverts commit b6d2b054e8baaee53fd2d4854c63cbf0f2c6262a.
> 
> blk-mq will no longer call the I/O scheduler .finish_request() callback
> for requests that were never inserted to the I/O scheduler.
> 
> Therefore, we can remove the logic inside mq-deadline that was added to
> workaround the (no longer existing) quirky behavior of blk-mq.
> 
> Signed-off-by: Niklas Cassel <niklas.cassel@....com>
> ---
>   block/mq-deadline.c | 16 +++++-----------
>   1 file changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> index 7f3c3932b723..b2d1e3adcb39 100644
> --- a/block/mq-deadline.c
> +++ b/block/mq-deadline.c
> @@ -678,7 +678,6 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>   
>   	prio = ioprio_class_to_prio[ioprio_class];
>   	dd_count(dd, inserted, prio);
> -	rq->elv.priv[0] = (void *)(uintptr_t)1;
>   
>   	if (blk_mq_sched_try_insert_merge(q, rq, &free)) {
>   		blk_mq_free_requests(&free);
> @@ -727,10 +726,12 @@ static void dd_insert_requests(struct blk_mq_hw_ctx *hctx,
>   	spin_unlock(&dd->lock);
>   }
>   
> -/* Callback from inside blk_mq_rq_ctx_init(). */
> +/*
> + * Nothing to do here. This is defined only to ensure that .finish_request
> + * method is called upon request completion.
> + */
>   static void dd_prepare_request(struct request *rq)
>   {
> -	rq->elv.priv[0] = NULL;
>   }

Please take a look at
https://lore.kernel.org/linux-block/18594aff-4a94-8a48-334c-f21ae32120c6@acm.org/
If dd_prepare_request() is removed I will have to reintroduce it.

Thanks,

Bart.