linux-kernel - Re: [PATCH V5 7/7] blk-mq-sched: don't dequeue request until all in ->dispatch are flushed

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Date:   Tue, 3 Oct 2017 02:11:28 -0700
From:   Christoph Hellwig <hch@...radead.org>
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 V5 7/7] blk-mq-sched: don't dequeue request until all in
 ->dispatch are flushed

This looks good in general:

Reviewed-by: Christoph Hellwig <hch@....de>

Minor nitpicks below:

>  	const bool has_sched_dispatch = e && e->type->ops.mq.dispatch_request;

This is now only tested once, so you can remove the local variable
for it.

> +		/*
> +		 * We may clear DISPATCH_BUSY just after it
> +		 * is set from another context, the only cost
> +		 * is that one request is dequeued a bit early,
> +		 * we can survive that. Given the window is
> +		 * small enough, no need to worry about performance
> +		 * effect.
> +		 */

Use your 80 line real estate for comments please.

>  	if (!has_sched_dispatch)
> +		if (!q->queue_depth) {
> +			blk_mq_flush_busy_ctxs(hctx, &rq_list);
> +			blk_mq_dispatch_rq_list(q, &rq_list);
> +		} else {
> +			blk_mq_do_dispatch_ctx(q, hctx);
> +		}
> +	} else {
>  		blk_mq_do_dispatch_sched(q, e, hctx);
> +	}

Maybe flatten this out to:

	if (e && e->type->ops.mq.dispatch_request) {
		blk_mq_do_dispatch_sched(q, e, hctx);
	} else if (q->queue_depth) {
		blk_mq_do_dispatch_ctx(q, hctx);
	} else {
		blk_mq_flush_busy_ctxs(hctx, &rq_list);
		blk_mq_dispatch_rq_list(q, &rq_list);
	}