blk-mq-sched: ask scheduler for work, if we failed dispatching leftovers

Usually we don't ask the scheduler for work, if we already have
leftovers on the dispatch list. This is done to leave work on
the scheduler side for as long as possible, for proper merging.
But if we do have work leftover but didn't dispatch anything,
then we should ask the scheduler since we could potentially
issue requests from that.
Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>

block/blk-mq-sched.c

@@ -175,6 +175,8 @@ void blk_mq_sched_put_request(struct request *rq)
 void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
 {
 	struct elevator_queue *e = hctx->queue->elevator;
+	const bool has_sched_dispatch = e && e->type->ops.mq.dispatch_request;
+	bool did_work = false;
 	LIST_HEAD(rq_list);
 
 	if (unlikely(blk_mq_hctx_stopped(hctx)))
@@ -204,11 +206,18 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
 	 */
 	if (!list_empty(&rq_list)) {
 		blk_mq_sched_mark_restart(hctx);
-		blk_mq_dispatch_rq_list(hctx, &rq_list);
-	} else if (!e || !e->type->ops.mq.dispatch_request) {
+		did_work = blk_mq_dispatch_rq_list(hctx, &rq_list);
+	} else if (!has_sched_dispatch) {
 		blk_mq_flush_busy_ctxs(hctx, &rq_list);
 		blk_mq_dispatch_rq_list(hctx, &rq_list);
-	} else {
+	}
+
+	/*
+	 * We want to dispatch from the scheduler if we had no work left
+	 * on the dispatch list, OR if we did have work but weren't able
+	 * to make progress.
+	 */
+	if (!did_work && has_sched_dispatch) {
 		do {
 			struct request *rq;