linux-kernel - Re: [PATCH V6 1/5] blk-mq-sched: fix scheduler bad performance

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Date:   Tue, 10 Oct 2017 11:10:22 -0700
From:   Omar Sandoval <osandov@...ndov.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...com>, linux-block@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Laurence Oberman <loberman@...hat.com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Tom Nguyen <tom81094@...il.com>, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, Omar Sandoval <osandov@...com>
Subject: Re: [PATCH V6 1/5] blk-mq-sched: fix scheduler bad performance

On Mon, Oct 09, 2017 at 07:24:20PM +0800, Ming Lei wrote:
> When hw queue is busy, we shouldn't take requests from
> scheduler queue any more, otherwise it is difficult to do
> IO merge.
> 
> This patch fixes the awful IO performance on some
> SCSI devices(lpfc, qla2xxx, ...) when mq-deadline/kyber
> is used by not taking requests if hw queue is busy.
> 
> Reviewed-by: Bart Van Assche <bart.vanassche@....com>
> Reviewed-by: Christoph Hellwig <hch@....de>

Reviewed-by: Omar Sandoval <osandov@...com>

> Signed-off-by: Ming Lei <ming.lei@...hat.com>
> ---
>  block/blk-mq-sched.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 4ab69435708c..eca011fdfa0e 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -94,7 +94,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
>  	struct request_queue *q = hctx->queue;
>  	struct elevator_queue *e = q->elevator;
>  	const bool has_sched_dispatch = e && e->type->ops.mq.dispatch_request;
> -	bool did_work = false;
> +	bool do_sched_dispatch = true;
>  	LIST_HEAD(rq_list);
>  
>  	/* RCU or SRCU read lock is needed before checking quiesced flag */
> @@ -125,18 +125,18 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
>  	 */
>  	if (!list_empty(&rq_list)) {
>  		blk_mq_sched_mark_restart_hctx(hctx);
> -		did_work = blk_mq_dispatch_rq_list(q, &rq_list);
> +		do_sched_dispatch = blk_mq_dispatch_rq_list(q, &rq_list);
>  	} else if (!has_sched_dispatch) {
>  		blk_mq_flush_busy_ctxs(hctx, &rq_list);
>  		blk_mq_dispatch_rq_list(q, &rq_list);
>  	}
>  
>  	/*
> -	 * We want to dispatch from the scheduler if we had no work left
> -	 * on the dispatch list, OR if we did have work but weren't able
> -	 * to make progress.
> +	 * We want to dispatch from the scheduler if there was nothing
> +	 * on the dispatch list or we were able to dispatch from the
> +	 * dispatch list.
>  	 */
> -	if (!did_work && has_sched_dispatch) {
> +	if (do_sched_dispatch && has_sched_dispatch) {
>  		do {
>  			struct request *rq;
>  
> -- 
> 2.9.5
>