blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request feedback

blk_insert_cloned_request() is called in the fast path of a dm-rq driver
(e.g. blk-mq request-based DM mpath).  blk_insert_cloned_request() uses
blk_mq_request_bypass_insert() to directly append the request to the
blk-mq hctx->dispatch_list of the underlying queue.

1) This way isn't efficient enough because the hctx spinlock is always
used.

2) With blk_insert_cloned_request(), we completely bypass underlying
queue's elevator and depend on the upper-level dm-rq driver's elevator
to schedule IO.  But dm-rq currently can't get the underlying queue's
dispatch feedback at all.  Without knowing whether a request was issued
or not (e.g. due to underlying queue being busy) the dm-rq elevator will
not be able to provide effective IO merging (as a side-effect of dm-rq
currently blindly destaging a request from its elevator only to requeue
it after a delay, which kills any opportunity for merging).  This
obviously causes very bad sequential IO performance.

Fix this by updating blk_insert_cloned_request() to use
blk_mq_request_direct_issue().  blk_mq_request_direct_issue() allows a
request to be issued directly to the underlying queue and returns the
dispatch feedback (blk_status_t).  If blk_mq_request_direct_issue()
returns BLK_SYS_RESOURCE the dm-rq driver will now use DM_MAPIO_REQUEUE
to _not_ destage the request.  Whereby preserving the opportunity to
merge IO.

With this, request-based DM's blk-mq sequential IO performance is vastly
improved (as much as 3X in mpath/virtio-scsi testing).

Signed-off-by: Ming Lei <ming.lei@redhat.com>
[blk-mq.c changes heavily influenced by Ming Lei's initial solution, but
they were refactored to make them less fragile and easier to read/review]
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -2500,8 +2500,7 @@ blk_status_t blk_insert_cloned_request(struct request_queue *q, struct request *
 		 * bypass a potential scheduler on the bottom device for
 		 * insert.
 		 */
-		blk_mq_request_bypass_insert(rq, true);
+		return blk_mq_request_direct_issue(rq);
-		return BLK_STS_OK;
 	}
 
 	spin_lock_irqsave(q->queue_lock, flags);

block/blk-mq.c

@@ -1775,15 +1775,19 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
 
 static void __blk_mq_fallback_to_insert(struct blk_mq_hw_ctx *hctx,
 					struct request *rq,
-					bool run_queue)
+					bool run_queue, bool bypass_insert)
 {
-	blk_mq_sched_insert_request(rq, false, run_queue, false,
+	if (!bypass_insert)
-					hctx->flags & BLK_MQ_F_BLOCKING);
+		blk_mq_sched_insert_request(rq, false, run_queue, false,
+					    hctx->flags & BLK_MQ_F_BLOCKING);
+	else
+		blk_mq_request_bypass_insert(rq, run_queue);
 }
 
 static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 						struct request *rq,
-						blk_qc_t *cookie)
+						blk_qc_t *cookie,
+						bool bypass_insert)
 {
 	struct request_queue *q = rq->q;
 	bool run_queue = true;
@@ -1794,7 +1798,7 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 		goto insert;
 	}
 
-	if (q->elevator)
+	if (q->elevator && !bypass_insert)
 		goto insert;
 
 	if (!blk_mq_get_driver_tag(rq, NULL, false))
@@ -1807,7 +1811,9 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 
 	return __blk_mq_issue_directly(hctx, rq, cookie);
 insert:
-	__blk_mq_fallback_to_insert(hctx, rq, run_queue);
+	__blk_mq_fallback_to_insert(hctx, rq, run_queue, bypass_insert);
+	if (bypass_insert)
+		return BLK_STS_RESOURCE;
 
 	return BLK_STS_OK;
 }
@@ -1822,15 +1828,30 @@ static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 
 	hctx_lock(hctx, &srcu_idx);
 
-	ret = __blk_mq_try_issue_directly(hctx, rq, cookie);
+	ret = __blk_mq_try_issue_directly(hctx, rq, cookie, false);
 	if (ret == BLK_STS_RESOURCE)
-		__blk_mq_fallback_to_insert(hctx, rq, true);
+		__blk_mq_fallback_to_insert(hctx, rq, true, false);
 	else if (ret != BLK_STS_OK)
 		blk_mq_end_request(rq, ret);
 
 	hctx_unlock(hctx, srcu_idx);
 }
 
+blk_status_t blk_mq_request_direct_issue(struct request *rq)
+{
+	blk_status_t ret;
+	int srcu_idx;
+	blk_qc_t unused_cookie;
+	struct blk_mq_ctx *ctx = rq->mq_ctx;
+	struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(rq->q, ctx->cpu);
+
+	hctx_lock(hctx, &srcu_idx);
+	ret = __blk_mq_try_issue_directly(hctx, rq, &unused_cookie, true);
+	hctx_unlock(hctx, srcu_idx);
+
+	return ret;
+}
+
 static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 {
 	const int is_sync = op_is_sync(bio->bi_opf);

block/blk-mq.h

@@ -74,6 +74,9 @@ void blk_mq_request_bypass_insert(struct request *rq, bool run_queue);
 void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
 				struct list_head *list);
 
+/* Used by blk_insert_cloned_request() to issue request directly */
+blk_status_t blk_mq_request_direct_issue(struct request *rq);
+
 /*
  * CPU -> queue mappings
  */

drivers/md/dm-rq.c

@@ -395,7 +395,7 @@ static void end_clone_request(struct request *clone, blk_status_t error)
 	dm_complete_request(tio->orig, error);
 }
 
-static void dm_dispatch_clone_request(struct request *clone, struct request *rq)
+static blk_status_t dm_dispatch_clone_request(struct request *clone, struct request *rq)
 {
 	blk_status_t r;
 
@@ -404,9 +404,10 @@ static void dm_dispatch_clone_request(struct request *clone, struct request *rq)
 
 	clone->start_time = jiffies;
 	r = blk_insert_cloned_request(clone->q, clone);
-	if (r)
+	if (r != BLK_STS_OK && r != BLK_STS_RESOURCE)
 		/* must complete clone in terms of original request */
 		dm_complete_request(rq, r);
+	return r;
 }
 
 static int dm_rq_bio_constructor(struct bio *bio, struct bio *bio_orig,
@@ -476,8 +477,10 @@ static int map_request(struct dm_rq_target_io *tio)
 	struct mapped_device *md = tio->md;
 	struct request *rq = tio->orig;
 	struct request *clone = NULL;
+	blk_status_t ret;
 
 	r = ti->type->clone_and_map_rq(ti, rq, &tio->info, &clone);
+check_again:
 	switch (r) {
 	case DM_MAPIO_SUBMITTED:
 		/* The target has taken the I/O to submit by itself later */
@@ -492,7 +495,17 @@ static int map_request(struct dm_rq_target_io *tio)
 		/* The target has remapped the I/O so dispatch it */
 		trace_block_rq_remap(clone->q, clone, disk_devt(dm_disk(md)),
 				     blk_rq_pos(rq));
-		dm_dispatch_clone_request(clone, rq);
+		ret = dm_dispatch_clone_request(clone, rq);
+		if (ret == BLK_STS_RESOURCE) {
+			blk_rq_unprep_clone(clone);
+			tio->ti->type->release_clone_rq(clone);
+			tio->clone = NULL;
+			if (!rq->q->mq_ops)
+				r = DM_MAPIO_DELAY_REQUEUE;
+			else
+				r = DM_MAPIO_REQUEUE;
+			goto check_again;
+		}
 		break;
 	case DM_MAPIO_REQUEUE:
 		/* The target wants to requeue the I/O */