Re: [PATCH V2 05/20] blk-mq-sched: improve dispatching from sw queue

On Tue, Aug 22, 2017 at 08:57:03PM +0000, Bart Van Assche wrote:

On Sat, 2017-08-05 at 14:56 +0800, Ming Lei wrote:

quoted

+static inline void blk_mq_do_dispatch_ctx(struct request_queue *q,
+					  struct blk_mq_hw_ctx *hctx)
+{
+	LIST_HEAD(rq_list);
+	struct blk_mq_ctx *ctx = NULL;
+
+	do {
+		struct request *rq;
+
+		rq = blk_mq_dispatch_rq_from_ctx(hctx, ctx);
+		if (!rq)
+			break;
+		list_add(&rq->queuelist, &rq_list);
+
+		/* round robin for fair dispatch */
+		ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
+	} while (blk_mq_dispatch_rq_list(q, &rq_list));
+}

An additional question about this patch: shouldn't request dequeuing start
at the software queue next to the last one from which a request got dequeued
instead of always starting at the first software queue (struct blk_mq_ctx
*ctx = NULL) to be truly round robin?

Looks a good idea, will introduce a ctx hint in hctx for starting when
blk_mq_dispatch_rq_list() returns zero. No lock is needed since
it is just a hint.

-- 
Ming