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

[Christoph Hellwig]

This looks good in general:

Reviewed-by: Christoph Hellwig <hch@xxxxxx>

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);
	}