Re: [PATCH 3/3] block: avoid hctx spinlock for plug with multiple queues

[Christoph Hellwig <hch@infradead.org>]

> +static void blk_mq_extract_queue_requests(struct rq_list *rqs,
> +					  struct rq_list *queue_rqs,
> +					  unsigned *queue_depth)
> +{
> +	struct rq_list matched_rqs = {}, unmatched_rqs = {};
> +	struct request *rq = rq_list_pop(rqs);
> +	struct request_queue *this_q = rq->q;
> +	unsigned depth = 1;
> +
> +	rq_list_add_tail(&matched_rqs, rq);
> +	while ((rq = rq_list_pop(rqs))) {
> +		if (rq->q == this_q) {
> +			rq_list_add_tail(&matched_rqs, rq);
> +			depth++;
> +		} else {
> +			rq_list_add_tail(&unmatched_rqs, rq);
> +		}

This might be moe efficient if you keep an extra iterator and never
mode the request for another queue.

> +	}
> +
> +	*queue_rqs = matched_rqs;
> +	*rqs = unmatched_rqs;
> +	*queue_depth = depth;

.. and I'd return the queue depth here instead of making it another
output argument.

> +static void blk_mq_dispatch_multiple_queue_requests(struct rq_list *rqs)
> +{
> +	do {
> +		struct rq_list queue_rqs;
> +		unsigned depth;
> +
> +		blk_mq_extract_queue_requests(rqs, &queue_rqs, &depth);
> +		blk_mq_dispatch_queue_requests(&queue_rqs, depth);
> +		while (!rq_list_empty(&queue_rqs)) {
> +			blk_mq_dispatch_list(&queue_rqs, false);
> +		}

No need for the braces in the inner while loop here.

The other caller of blk_mq_dispatch_list loops until the list is empty,
why don't we need that here?