blk-mq: introduce .get_budget and .put_budget in blk_mq_ops

For SCSI devices, there is often a per-request-queue depth, which needs
to be respected before queuing one request.

Currently blk-mq always dequeues the request first, then calls
.queue_rq() to dispatch the request to lld. One obvious issue with this
approach is that I/O merging may not be successful, because when the
per-request-queue depth can't be respected, .queue_rq() has to return
BLK_STS_RESOURCE, and then this request has to stay in hctx->dispatch
list. This means it never gets a chance to be merged with other IO.

This patch introduces .get_budget and .put_budget callback in blk_mq_ops,
then we can try to get reserved budget first before dequeuing request.
If the budget for queueing I/O can't be satisfied, we don't need to
dequeue request at all. Hence the request can be left in the IO
scheduler queue, for more merging opportunities.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 38 insertions, 5 deletions

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 40cba1b..dcb4673 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1048,7 +1048,8 @@ static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx *hctx)
 	return true;
 }
 
-bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
+bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
+		bool got_budget)
 {
 	struct blk_mq_hw_ctx *hctx;
 	struct request *rq;
@@ -1057,6 +1058,8 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
 	if (list_empty(list))
 		return false;
 
+	WARN_ON(!list_is_singular(list) && got_budget);
+
 	/*
 	 * Now process all the entries, sending them to the driver.
 	 */
@@ -1074,16 +1077,30 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
 			 * The initial allocation attempt failed, so we need to
 			 * rerun the hardware queue when a tag is freed.
 			 */
-			if (!blk_mq_dispatch_wait_add(hctx))
+			if (!blk_mq_dispatch_wait_add(hctx)) {
+				if (got_budget)
+					blk_mq_put_dispatch_budget(hctx);
 				break;
+			}
 
 			/*
 			 * It's possible that a tag was freed in the window
 			 * between the allocation failure and adding the
 			 * hardware queue to the wait queue.
 			 */
-			if (!blk_mq_get_driver_tag(rq, &hctx, false))
+			if (!blk_mq_get_driver_tag(rq, &hctx, false)) {
+				if (got_budget)
+					blk_mq_put_dispatch_budget(hctx);
+				break;
+			}
+		}
+
+		if (!got_budget) {
+			ret = blk_mq_get_dispatch_budget(hctx);
+			if (ret == BLK_STS_RESOURCE)
 				break;
+			if (ret != BLK_STS_OK)
+				goto fail_rq;
 		}
 
 		list_del_init(&rq->queuelist);
@@ -1111,6 +1128,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
 			break;
 		}
 
+ fail_rq:
 		if (unlikely(ret != BLK_STS_OK)) {
 			errors++;
 			blk_mq_end_request(rq, BLK_STS_IOERR);
@@ -1169,6 +1187,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
 static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 {
 	int srcu_idx;
+	bool run_queue;
 
 	/*
 	 * We should be running this queue from one of the CPUs that
@@ -1185,15 +1204,18 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 
 	if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
 		rcu_read_lock();
-		blk_mq_sched_dispatch_requests(hctx);
+		run_queue = blk_mq_sched_dispatch_requests(hctx);
 		rcu_read_unlock();
 	} else {
 		might_sleep();
 
 		srcu_idx = srcu_read_lock(hctx->queue_rq_srcu);
-		blk_mq_sched_dispatch_requests(hctx);
+		run_queue = blk_mq_sched_dispatch_requests(hctx);
 		srcu_read_unlock(hctx->queue_rq_srcu, srcu_idx);
 	}
+
+	if (run_queue)
+		blk_mq_run_hw_queue(hctx, true);
 }
 
 /*
@@ -1582,6 +1604,13 @@ static void __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 	if (!blk_mq_get_driver_tag(rq, NULL, false))
 		goto insert;
 
+	ret = blk_mq_get_dispatch_budget(hctx);
+	if (ret == BLK_STS_RESOURCE) {
+		blk_mq_put_driver_tag(rq);
+		goto insert;
+	} else if (ret != BLK_STS_OK)
+		goto fail_rq;
+
 	new_cookie = request_to_qc_t(hctx, rq);
 
 	/*
@@ -1598,6 +1627,7 @@ static void __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 		__blk_mq_requeue_request(rq);
 		goto insert;
 	default:
+ fail_rq:
 		*cookie = BLK_QC_T_NONE;
 		blk_mq_end_request(rq, ret);
 		return;
@@ -2582,6 +2612,9 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
 	if (!set->ops->queue_rq)
 		return -EINVAL;
 
+	if (!set->ops->get_budget ^ !set->ops->put_budget)
+		return -EINVAL;
+
 	if (set->queue_depth > BLK_MQ_MAX_DEPTH) {
 		pr_info("blk-mq: reduced tag depth to %u\n",
 			BLK_MQ_MAX_DEPTH);