linux-kernel - Re: [PATCH BUGFIX/IMPROVEMENT 2/6] block, bfq: put reqs of waker and woken in dispatch list

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Date:   Tue, 26 Jan 2021 09:18:49 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Paolo Valente <paolo.valente@...aro.org>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.cz>
Subject: Re: [PATCH BUGFIX/IMPROVEMENT 2/6] block, bfq: put reqs of waker and
 woken in dispatch list

On 1/26/21 3:50 AM, Paolo Valente wrote:
> Consider a new I/O request that arrives for a bfq_queue bfqq. If, when
> this happens, the only active bfq_queues are bfqq and either its waker
> bfq_queue or one of its woken bfq_queues, then there is no point in
> queueing this new I/O request in bfqq for service. In fact, the
> in-service queue and bfqq agree on serving this new I/O request as
> soon as possible. So this commit puts this new I/O request directly
> into the dispatch list.
> 
> Tested-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
> ---
>  block/bfq-iosched.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index a83149407336..e5b83910fbe0 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -5640,7 +5640,22 @@ static void bfq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>  
>  	spin_lock_irq(&bfqd->lock);
>  	bfqq = bfq_init_rq(rq);
> -	if (!bfqq || at_head || blk_rq_is_passthrough(rq)) {
> +
> +	/*
> +	 * Additional case for putting rq directly into the dispatch
> +	 * queue: the only active bfq_queues are bfqq and either its
> +	 * waker bfq_queue or one of its woken bfq_queues. In this
> +	 * case, there is no point in queueing rq in bfqq for
> +	 * service. In fact, the in-service queue and bfqq agree on
> +	 * serving this new I/O request as soon as possible.
> +	 */
> +	if (!bfqq ||
> +	    (bfqq != bfqd->in_service_queue &&
> +	     bfqd->in_service_queue != NULL &&
> +	     bfq_tot_busy_queues(bfqd) == 1 + bfq_bfqq_busy(bfqq) &&
> +	     (bfqq->waker_bfqq == bfqd->in_service_queue ||
> +	      bfqd->in_service_queue->waker_bfqq == bfqq)) ||
> +	    at_head || blk_rq_is_passthrough(rq)) {
>  		if (at_head)
>  			list_add(&rq->queuelist, &bfqd->dispatch);
>  		else
> 

This is unreadable... Just seems like you are piling heuristics in to
catch some case, and it's neither readable nor clean.

-- 
Jens Axboe