linux-kernel - Re: [PATCH 04/14] blk-mq: pass in request/bio flags to queue mapping

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Date:   Mon, 29 Oct 2018 10:30:36 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/14] blk-mq: pass in request/bio flags to queue mapping

On Mon, 2018-10-29 at 10:37 -0600, Jens Axboe wrote:
> @@ -400,9 +402,15 @@ void blk_mq_sched_insert_requests(struct request_queue *q,
>  				  struct blk_mq_ctx *ctx,
>  				  struct list_head *list, bool run_queue_async)
>  {
> -	struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);
> -	struct elevator_queue *e = hctx->queue->elevator;
> +	struct blk_mq_hw_ctx *hctx;
> +	struct elevator_queue *e;
> +	struct request *rq;
> +
> +	/* For list inserts, requests better be on the same hw queue */
> +	rq = list_first_entry(list, struct request, queuelist);
> +	hctx = blk_mq_map_queue(q, rq->cmd_flags, ctx->cpu);

Passing all request cmd_flags bits to blk_mq_map_queue() makes it possible
for that function to depend on every single cmd_flags bit even if different
requests have different cmd_flags. Have you considered to pass the hw_ctx
type only to blk_mq_map_queue() to avoid that that function would start
depending on other cmd_flags?

Additionally, what guarantees that all requests in queuelist have the same
hw_ctx type? If a later patch will guarantee that, please mention that in
the comment about list_first_entry().

Thanks,

Bart.