[PATCH V7 4/6] blk-mq: introduce .get_budget and .put_budget in blk_mq_ops

[Ming Lei <ming.lei@redhat.com>]

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

The current blk-mq always dequeues one request first, then calls .queue_rq()
to dispatch the request to lld. One obvious issue of this way is that I/O
merge may not be good, because when the per-request-queue depth can't be
respected,  .queue_rq() has to return BLK_STS_RESOURCE, then this request
has to staty in hctx->dispatch list, and never got chance to participate
into I/O merge.

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.
Once we can't get budget for queueing I/O, we don't need to dequeue request
at all, then I/O merge can get improved a lot.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq-sched.c   | 39 +++++++++++++++++++++++++++++++--------
 block/blk-mq.c         | 35 ++++++++++++++++++++++++++++++++---
 block/blk-mq.h         |  2 +-
 include/linux/blk-mq.h | 10 ++++++++++
 4 files changed, 74 insertions(+), 12 deletions(-)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index be29ba849408..bcf4773ee1ca 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -89,19 +89,32 @@ static bool blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)
 	return false;
 }
 
-static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
+static bool blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
 {
 	struct request_queue *q = hctx->queue;
 	struct elevator_queue *e = q->elevator;
 	LIST_HEAD(rq_list);
 
 	do {
-		struct request *rq = e->type->ops.mq.dispatch_request(hctx);
+		struct request *rq;
 
-		if (!rq)
+		if (e->type->ops.mq.has_work &&
+				!e->type->ops.mq.has_work(hctx))
 			break;
+
+		if (q->mq_ops->get_budget && !q->mq_ops->get_budget(hctx))
+			return true;
+
+		rq = e->type->ops.mq.dispatch_request(hctx);
+		if (!rq) {
+			if (q->mq_ops->put_budget)
+				q->mq_ops->put_budget(hctx);
+			break;
+		}
 		list_add(&rq->queuelist, &rq_list);
-	} while (blk_mq_dispatch_rq_list(q, &rq_list));
+	} while (blk_mq_dispatch_rq_list(q, &rq_list, true));
+
+	return false;
 }
 
 void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
@@ -110,6 +123,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
 	struct elevator_queue *e = q->elevator;
 	const bool has_sched_dispatch = e && e->type->ops.mq.dispatch_request;
 	LIST_HEAD(rq_list);
+	bool run_queue = false;
 
 	/* RCU or SRCU read lock is needed before checking quiesced flag */
 	if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
@@ -143,13 +157,22 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
 	 */
 	if (!list_empty(&rq_list)) {
 		blk_mq_sched_mark_restart_hctx(hctx);
-		if (blk_mq_dispatch_rq_list(q, &rq_list) && has_sched_dispatch)
-			blk_mq_do_dispatch_sched(hctx);
+		if (blk_mq_dispatch_rq_list(q, &rq_list, false) &&
+				has_sched_dispatch)
+			run_queue = blk_mq_do_dispatch_sched(hctx);
 	} else if (has_sched_dispatch) {
-		blk_mq_do_dispatch_sched(hctx);
+		run_queue = blk_mq_do_dispatch_sched(hctx);
 	} else {
 		blk_mq_flush_busy_ctxs(hctx, &rq_list);
-		blk_mq_dispatch_rq_list(q, &rq_list);
+		blk_mq_dispatch_rq_list(q, &rq_list, false);
+	}
+
+	if (run_queue) {
+		if (!blk_mq_sched_needs_restart(hctx) &&
+		    !test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state)) {
+			blk_mq_sched_mark_restart_hctx(hctx);
+			blk_mq_run_hw_queue(hctx, true);
+		}
 	}
 }
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 076cbab9c3e0..e6e50a91f170 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1045,7 +1045,16 @@ 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)
+static void blk_mq_put_budget(struct blk_mq_hw_ctx *hctx, bool got_budget)
+{
+	struct request_queue *q = hctx->queue;
+
+	if (q->mq_ops->put_budget && got_budget)
+		q->mq_ops->put_budget(hctx);
+}
+
+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;
@@ -1054,6 +1063,8 @@ 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.
 	 */
@@ -1071,15 +1082,25 @@ 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)) {
+				blk_mq_put_budget(hctx, got_budget);
 				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)) {
+				blk_mq_put_budget(hctx, got_budget);
+				break;
+			}
+		}
+
+		if (!got_budget) {
+			if (q->mq_ops->get_budget &&
+					!q->mq_ops->get_budget(hctx))
 				break;
 		}
 
@@ -1577,6 +1598,11 @@ static void __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 	if (!blk_mq_get_driver_tag(rq, NULL, false))
 		goto insert;
 
+	if (q->mq_ops->get_budget && !q->mq_ops->get_budget(hctx)) {
+		blk_mq_put_driver_tag(rq);
+		goto insert;
+	}
+
 	new_cookie = request_to_qc_t(hctx, rq);
 
 	/*
@@ -2577,6 +2603,9 @@ 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);
diff --git a/block/blk-mq.h b/block/blk-mq.h
index ef15b3414da5..d1bfce3e69ad 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -30,7 +30,7 @@ void blk_mq_freeze_queue(struct request_queue *q);
 void blk_mq_free_queue(struct request_queue *q);
 int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr);
 void blk_mq_wake_waiters(struct request_queue *q);
-bool blk_mq_dispatch_rq_list(struct request_queue *, struct list_head *);
+bool blk_mq_dispatch_rq_list(struct request_queue *, struct list_head *, bool);
 void blk_mq_flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head *list);
 bool blk_mq_hctx_has_pending(struct blk_mq_hw_ctx *hctx);
 bool blk_mq_get_driver_tag(struct request *rq, struct blk_mq_hw_ctx **hctx,
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 50c6485cb04f..f8c4ba2682ec 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -90,6 +90,8 @@ struct blk_mq_queue_data {
 
 typedef blk_status_t (queue_rq_fn)(struct blk_mq_hw_ctx *,
 		const struct blk_mq_queue_data *);
+typedef bool (get_budget_fn)(struct blk_mq_hw_ctx *);
+typedef void (put_budget_fn)(struct blk_mq_hw_ctx *);
 typedef enum blk_eh_timer_return (timeout_fn)(struct request *, bool);
 typedef int (init_hctx_fn)(struct blk_mq_hw_ctx *, void *, unsigned int);
 typedef void (exit_hctx_fn)(struct blk_mq_hw_ctx *, unsigned int);
@@ -112,6 +114,14 @@ struct blk_mq_ops {
 	queue_rq_fn		*queue_rq;
 
 	/*
+	 * Reserve budget before queue request, once .queue_rq is
+	 * run, it is driver's responsibility to release the
+	 * reserved budget.
+	 */
+	get_budget_fn		*get_budget;
+	put_budget_fn		*put_budget;
+
+	/*
 	 * Called on request timeout
 	 */
 	timeout_fn		*timeout;
-- 
2.9.5