diff mbox

[PATCHSET,v4] blk-mq-scheduling framework

Message ID 1482423775.2682.6.camel@sandisk.com
State New, archived
Headers show

Commit Message

Bart Van Assche Dec. 22, 2016, 4:23 p.m. UTC
On Fri, 2016-12-16 at 17:12 -0700, Jens Axboe wrote:
> From the discussion last time, I looked into the feasibility of having
> two sets of tags for the same request pool, to avoid having to copy
> some of the request fields at dispatch and completion time. To do that,
> we'd have to replace the driver tag map(s) with our own, and augment
> that with tag map(s) on the side representing the device queue depth.
> Queuing IO with the scheduler would allocate from the new map, and
> dispatching would acquire the "real" tag. We would need to change
> drivers to do this, or add an extra indirection table to map a real
> tag to the scheduler tag. We would also need a 1:1 mapping between
> scheduler and hardware tag pools, or additional info to track it.
> Unless someone can convince me otherwise, I think the current approach
> is cleaner.

Hello Jens,

Can you have a look at the attached patches? These implement the "two tags
per request" approach without a table that maps one tag type to the other
or any other ugly construct. __blk_mq_alloc_request() is modified such that
it assigns rq->sched_tag and sched_tags->rqs[] instead of rq->tag and
tags->rqs[]. rq->tag and tags->rqs[] are assigned just before dispatch by
blk_mq_assign_drv_tag(). This approach results in significantly less code
than the approach proposed in v4 of your blk-mq-sched patch series. Memory
usage is lower because only a single set of requests is allocated. The
runtime overhead is lower because request fields no longer have to be
copied between the requests owned by the block driver and the requests
owned by the I/O scheduler. I can boot a VM from the virtio-blk driver but
otherwise the attached patches have not yet been tested.

Thanks,

Bart.

Comments

Omar Sandoval Dec. 22, 2016, 4:52 p.m. UTC | #1
On Thu, Dec 22, 2016 at 04:23:24PM +0000, Bart Van Assche wrote:
> On Fri, 2016-12-16 at 17:12 -0700, Jens Axboe wrote:
> > From the discussion last time, I looked into the feasibility of having
> > two sets of tags for the same request pool, to avoid having to copy
> > some of the request fields at dispatch and completion time. To do that,
> > we'd have to replace the driver tag map(s) with our own, and augment
> > that with tag map(s) on the side representing the device queue depth.
> > Queuing IO with the scheduler would allocate from the new map, and
> > dispatching would acquire the "real" tag. We would need to change
> > drivers to do this, or add an extra indirection table to map a real
> > tag to the scheduler tag. We would also need a 1:1 mapping between
> > scheduler and hardware tag pools, or additional info to track it.
> > Unless someone can convince me otherwise, I think the current approach
> > is cleaner.
> 
> Hello Jens,
> 
> Can you have a look at the attached patches? These implement the "two tags
> per request" approach without a table that maps one tag type to the other
> or any other ugly construct. __blk_mq_alloc_request() is modified such that
> it assigns rq->sched_tag and sched_tags->rqs[] instead of rq->tag and
> tags->rqs[]. rq->tag and tags->rqs[] are assigned just before dispatch by
> blk_mq_assign_drv_tag(). This approach results in significantly less code
> than the approach proposed in v4 of your blk-mq-sched patch series. Memory
> usage is lower because only a single set of requests is allocated. The
> runtime overhead is lower because request fields no longer have to be
> copied between the requests owned by the block driver and the requests
> owned by the I/O scheduler. I can boot a VM from the virtio-blk driver but
> otherwise the attached patches have not yet been tested.
> 
> Thanks,
> 
> Bart.

Hey, Bart,

This approach occurred to us, but we couldn't figure out a way to make
blk_mq_tag_to_rq() work with it. From skimming over the patches, I
didn't see a solution to that problem.
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Omar Sandoval Dec. 22, 2016, 5:12 p.m. UTC | #2
On Thu, Dec 22, 2016 at 04:57:36PM +0000, Bart Van Assche wrote:
> On Thu, 2016-12-22 at 08:52 -0800, Omar Sandoval wrote:
> > This approach occurred to us, but we couldn't figure out a way to make
> > blk_mq_tag_to_rq() work with it. From skimming over the patches, I
> > didn't see a solution to that problem.
> 
> Hello Omar,
> 
> Can you clarify your comment? Since my patches initialize both tags->rqs[]
> and sched_tags->rqs[] the function blk_mq_tag_to_rq() should still work.
> 
> Bart.

Sorry, you're right, it does work, but tags->rqs[] ends up being the
extra lookup table. I suspect that the runtime overhead of keeping that
up to date could be worse than copying the rq fields if you have lots of
CPUs but only one hardware queue.
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Bart Van Assche Dec. 22, 2016, 5:39 p.m. UTC | #3
On Thu, 2016-12-22 at 09:12 -0800, Omar Sandoval wrote:
> On Thu, Dec 22, 2016 at 04:57:36PM +0000, Bart Van Assche wrote:

> > On Thu, 2016-12-22 at 08:52 -0800, Omar Sandoval wrote:

> > > This approach occurred to us, but we couldn't figure out a way to make

> > > blk_mq_tag_to_rq() work with it. From skimming over the patches, I

> > > didn't see a solution to that problem.

> > 

> > Can you clarify your comment? Since my patches initialize both tags->rqs[]

> > and sched_tags->rqs[] the function blk_mq_tag_to_rq() should still work.

> 

> Sorry, you're right, it does work, but tags->rqs[] ends up being the

> extra lookup table. I suspect that the runtime overhead of keeping that

> up to date could be worse than copying the rq fields if you have lots of

> CPUs but only one hardware queue.


Hello Omar,

I'm not sure that anything can be done if the number of CPUs that is submitting
I/O is large compared to the queue depth so I don't think we should spend our
time on that case. If the queue depth is large enough then the sbitmap code will
allocate tags such that different CPUs use different rqs[] elements.

The advantages of the approach I proposed are such that I am convinced that is
what we should start from and address contention on the tags->rqs[] array if it
measurements show that it is necessary to address it.

Bart.
diff mbox

Patch

From c49ec4e8b0e4135a87c9894597901539f3e3ca08 Mon Sep 17 00:00:00 2001
From: Bart Van Assche <bart.vanassche@sandisk.com>
Date: Wed, 21 Dec 2016 12:39:33 +0100
Subject: [PATCH 3/3] blk-mq: Split driver and scheduler tags

Add 'sched_tags' next to 'tags' in struct blk_mq_hw_ctx and also
in struct blk_mq_tag_set. Add 'sched_tag' next to 'tag' in struct
request. Modify blk_mq_update_nr_requests() such that it accepts
values larger than the queue depth. Make __blk_mq_free_request()
free both tags. Make blk_mq_alloc_tag_set() allocate both tag sets.
Make blk_mq_free_tag_set() free both tag sets. Make
blk_mq_dispatch_rq_list() allocate the driver tag. Modify
blk_mq_update_nr_requests() such that it accepts a size that
exceeds the queue depth.
---
 block/blk-flush.c      |   9 ++-
 block/blk-mq.c         | 160 +++++++++++++++++++++++++++++++++++--------------
 block/blk-mq.h         |   5 +-
 block/blk-tag.c        |   1 +
 include/linux/blk-mq.h |   2 +
 include/linux/blkdev.h |   1 +
 6 files changed, 129 insertions(+), 49 deletions(-)

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 6a7c29d2eb3c..46d12bbfde85 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -170,6 +170,8 @@  static bool blk_flush_complete_seq(struct request *rq,
 	struct list_head *pending = &fq->flush_queue[fq->flush_pending_idx];
 	bool queued = false, kicked;
 
+	BUG_ON(rq->tag < 0);
+
 	BUG_ON(rq->flush.seq & seq);
 	rq->flush.seq |= seq;
 
@@ -319,6 +321,8 @@  static bool blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq)
 	if (q->mq_ops) {
 		struct blk_mq_hw_ctx *hctx;
 
+		BUG_ON(first_rq->tag < 0);
+
 		flush_rq->mq_ctx = first_rq->mq_ctx;
 		flush_rq->tag = first_rq->tag;
 		fq->orig_rq = first_rq;
@@ -452,8 +456,9 @@  void blk_insert_flush(struct request *rq)
 	 * processed directly without going through flush machinery.  Queue
 	 * for normal execution.
 	 */
-	if ((policy & REQ_FSEQ_DATA) &&
-	    !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
+	if (((policy & REQ_FSEQ_DATA) &&
+	     !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) ||
+	    (q->mq_ops && blk_mq_assign_drv_tag(rq) < 0)) {
 		if (q->mq_ops)
 			blk_mq_sched_insert_request(rq, false, true, false);
 		else
diff --git a/block/blk-mq.c b/block/blk-mq.c
index b68b7fc43e46..48d7968d4ed9 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -220,20 +220,21 @@  EXPORT_SYMBOL_GPL(blk_mq_rq_ctx_init);
 struct request *__blk_mq_alloc_request(struct blk_mq_alloc_data *data,
 				       unsigned int op)
 {
-	struct blk_mq_tags *tags = data->hctx->tags;
+	struct blk_mq_tags *tags = data->hctx->sched_tags;
 	struct request *rq;
-	unsigned int tag;
+	unsigned int sched_tag;
 
-	tag = blk_mq_get_tag(data, tags);
-	if (tag != BLK_MQ_TAG_FAIL) {
-		rq = tags->rqs[tag];
+	sched_tag = blk_mq_get_tag(data, tags);
+	if (sched_tag != BLK_MQ_TAG_FAIL) {
+		rq = tags->rqs[sched_tag];
+		rq->tag = -1;
 
 		if (blk_mq_tag_busy(data->hctx)) {
 			rq->rq_flags = RQF_MQ_INFLIGHT;
 			atomic_inc(&data->hctx->nr_active);
 		}
 
-		rq->tag = tag;
+		rq->sched_tag = sched_tag;
 		blk_mq_rq_ctx_init(data->q, data->ctx, rq, op);
 		return rq;
 	}
@@ -328,6 +329,7 @@  void __blk_mq_finish_request(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
 			     struct request *rq)
 {
 	const int tag = rq->tag;
+	const int sched_tag = rq->sched_tag;
 	struct request_queue *q = rq->q;
 
 	ctx->rq_completed[rq_is_sync(rq)]++;
@@ -340,7 +342,13 @@  void __blk_mq_finish_request(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
 
 	clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
 	clear_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags);
-	blk_mq_put_tag(hctx, hctx->tags, ctx, tag);
+	if (tag >= 0) {
+		WARN_ON_ONCE(hctx->tags->rqs[tag] != rq);
+		hctx->tags->rqs[tag] = NULL;
+		blk_mq_put_tag(hctx, hctx->tags, ctx, tag);
+	}
+	if (sched_tag >= 0)
+		blk_mq_put_tag(hctx, hctx->sched_tags, ctx, sched_tag);
 	blk_queue_exit(q);
 }
 
@@ -844,6 +852,26 @@  static inline unsigned int queued_to_index(unsigned int queued)
 	return min(BLK_MQ_MAX_DISPATCH_ORDER - 1, ilog2(queued) + 1);
 }
 
+int blk_mq_assign_drv_tag(struct request *rq)
+{
+	struct request_queue *q = rq->q;
+	struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, rq->mq_ctx->cpu);
+	struct blk_mq_alloc_data data = {
+		.q = rq->q,
+		.ctx = rq->mq_ctx,
+		.hctx = hctx,
+	};
+
+	rq->tag = blk_mq_get_tag(&data, hctx->tags);
+	if (rq->tag < 0)
+		goto out;
+	WARN_ON_ONCE(hctx->tags->rqs[rq->tag]);
+	hctx->tags->rqs[rq->tag] = rq;
+
+out:
+	return rq->tag;
+}
+
 bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list)
 {
 	struct request_queue *q = hctx->queue;
@@ -866,6 +894,8 @@  bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list)
 		struct blk_mq_queue_data bd;
 
 		rq = list_first_entry(list, struct request, queuelist);
+		if (rq->tag < 0 && blk_mq_assign_drv_tag(rq) < 0)
+			break;
 		list_del_init(&rq->queuelist);
 
 		bd.rq = rq;
@@ -1296,7 +1326,8 @@  static void blk_mq_try_issue_directly(struct request *rq, blk_qc_t *cookie)
 		goto insert;
 
 	hctx = blk_mq_map_queue(q, rq->mq_ctx->cpu);
-	if (blk_mq_hctx_stopped(hctx))
+	if (blk_mq_hctx_stopped(hctx) ||
+	    (rq->tag < 0 && blk_mq_assign_drv_tag(rq) < 0))
 		goto insert;
 
 	new_cookie = blk_tag_to_qc_t(rq->tag, hctx->queue_num);
@@ -1592,17 +1623,19 @@  void blk_mq_free_rq_map(struct blk_mq_tags *tags)
 }
 
 struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
-					unsigned int hctx_idx)
+					unsigned int hctx_idx,
+					unsigned int nr_tags,
+					unsigned int reserved_tags)
 {
 	struct blk_mq_tags *tags;
 
-	tags = blk_mq_init_tags(set->queue_depth, set->reserved_tags,
+	tags = blk_mq_init_tags(nr_tags, reserved_tags,
 				set->numa_node,
 				BLK_MQ_FLAG_TO_ALLOC_POLICY(set->flags));
 	if (!tags)
 		return NULL;
 
-	tags->rqs = kzalloc_node(set->queue_depth * sizeof(struct request *),
+	tags->rqs = kzalloc_node(nr_tags * sizeof(struct request *),
 				 GFP_NOIO | __GFP_NOWARN | __GFP_NORETRY,
 				 set->numa_node);
 	if (!tags->rqs) {
@@ -1800,6 +1833,7 @@  static int blk_mq_init_hctx(struct request_queue *q,
 	cpuhp_state_add_instance_nocalls(CPUHP_BLK_MQ_DEAD, &hctx->cpuhp_dead);
 
 	hctx->tags = set->tags[hctx_idx];
+	hctx->sched_tags = set->sched_tags[hctx_idx];
 
 	/*
 	 * Allocate space for all possible cpus to avoid allocation at
@@ -1881,6 +1915,38 @@  static void blk_mq_init_cpu_queues(struct request_queue *q,
 	}
 }
 
+static void __blk_mq_free_rq_map_i(struct blk_mq_tag_set *set, int hctx_idx)
+{
+	if (set->sched_tags[hctx_idx]) {
+		blk_mq_free_rqs(set, set->sched_tags[hctx_idx], hctx_idx);
+		blk_mq_free_rq_map(set->sched_tags[hctx_idx]);
+		set->sched_tags[hctx_idx] = NULL;
+	}
+	if (set->tags[hctx_idx]) {
+		blk_mq_free_rq_map(set->tags[hctx_idx]);
+		set->tags[hctx_idx] = NULL;
+	}
+}
+
+static bool __blk_mq_alloc_rq_map_i(struct blk_mq_tag_set *set, int hctx_idx,
+				    unsigned int nr_requests)
+{
+	int ret = 0;
+
+	set->tags[hctx_idx] = blk_mq_alloc_rq_map(set, hctx_idx,
+					set->queue_depth, set->reserved_tags);
+	set->sched_tags[hctx_idx] = blk_mq_alloc_rq_map(set, hctx_idx,
+							nr_requests, 0);
+	if (set->sched_tags[hctx_idx])
+		ret = blk_mq_alloc_rqs(set, set->sched_tags[hctx_idx],
+				       hctx_idx);
+	if (!set->tags[hctx_idx] || !set->sched_tags[hctx_idx] || ret < 0) {
+		__blk_mq_free_rq_map_i(set, hctx_idx);
+		return false;
+	}
+	return true;
+}
+
 static void blk_mq_map_swqueue(struct request_queue *q,
 			       const struct cpumask *online_mask)
 {
@@ -1909,23 +1975,15 @@  static void blk_mq_map_swqueue(struct request_queue *q,
 
 		hctx_idx = q->mq_map[i];
 		/* unmapped hw queue can be remapped after CPU topo changed */
-		if (!set->tags[hctx_idx]) {
-			set->tags[hctx_idx] = blk_mq_alloc_rq_map(set,
-								  hctx_idx);
-			if (blk_mq_alloc_rqs(set, set->tags[hctx_idx],
-					     hctx_idx) < 0) {
-				blk_mq_free_rq_map(set->tags[hctx_idx]);
-				set->tags[hctx_idx] = NULL;
-			}
-
+		if (!set->tags[hctx_idx] &&
+		    !__blk_mq_alloc_rq_map_i(set, hctx_idx, q->nr_requests)) {
 			/*
 			 * If tags initialization fail for some hctx,
 			 * that hctx won't be brought online.  In this
 			 * case, remap the current ctx to hctx[0] which
 			 * is guaranteed to always have tags allocated
 			 */
-			if (!set->tags[hctx_idx])
-				q->mq_map[i] = 0;
+			q->mq_map[i] = 0;
 		}
 
 		ctx = per_cpu_ptr(q->queue_ctx, i);
@@ -2318,26 +2376,20 @@  static int blk_mq_queue_reinit_prepare(unsigned int cpu)
 	return 0;
 }
 
-static int __blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)
+static int __blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set,
+				  unsigned int nr_requests)
 {
 	int i;
 
-	for (i = 0; i < set->nr_hw_queues; i++) {
-		set->tags[i] = blk_mq_alloc_rq_map(set, i);
-		if (!set->tags[i])
+	for (i = 0; i < set->nr_hw_queues; i++)
+		if (!__blk_mq_alloc_rq_map_i(set, i, nr_requests))
 			goto out_unwind;
-		if (blk_mq_alloc_rqs(set, set->tags[i], i) < 0)
-			goto free_rq_map;
-	}
 
 	return 0;
 
 out_unwind:
-	while (--i >= 0) {
-		blk_mq_free_rqs(set, set->tags[i], i);
-free_rq_map:
-		blk_mq_free_rq_map(set->tags[i]);
-	}
+	while (--i >= 0)
+		__blk_mq_free_rq_map_i(set, i);
 
 	return -ENOMEM;
 }
@@ -2347,14 +2399,15 @@  static int __blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)
  * may reduce the depth asked for, if memory is tight. set->queue_depth
  * will be updated to reflect the allocated depth.
  */
-static int blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)
+static int blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set,
+				unsigned int nr_requests)
 {
 	unsigned int depth;
 	int err;
 
 	depth = set->queue_depth;
 	do {
-		err = __blk_mq_alloc_rq_maps(set);
+		err = __blk_mq_alloc_rq_maps(set, nr_requests);
 		if (!err)
 			break;
 
@@ -2385,7 +2438,7 @@  static int blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)
  */
 int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
 {
-	int ret;
+	int ret = -ENOMEM;
 
 	BUILD_BUG_ON(BLK_MQ_MAX_DEPTH > 1 << BLK_MQ_UNIQUE_TAG_BITS);
 
@@ -2425,32 +2478,39 @@  int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
 	if (!set->tags)
 		return -ENOMEM;
 
-	ret = -ENOMEM;
+	set->sched_tags = kzalloc_node(nr_cpu_ids * sizeof(struct blk_mq_tags *),
+				       GFP_KERNEL, set->numa_node);
+	if (!set->sched_tags)
+		goto free_drv_tags;
+
 	set->mq_map = kzalloc_node(sizeof(*set->mq_map) * nr_cpu_ids,
 			GFP_KERNEL, set->numa_node);
 	if (!set->mq_map)
-		goto out_free_tags;
+		goto free_sched_tags;
 
 	if (set->ops->map_queues)
 		ret = set->ops->map_queues(set);
 	else
 		ret = blk_mq_map_queues(set);
 	if (ret)
-		goto out_free_mq_map;
+		goto free_mq_map;
 
-	ret = blk_mq_alloc_rq_maps(set);
+	ret = blk_mq_alloc_rq_maps(set, set->queue_depth/*q->nr_requests*/);
 	if (ret)
-		goto out_free_mq_map;
+		goto free_mq_map;
 
 	mutex_init(&set->tag_list_lock);
 	INIT_LIST_HEAD(&set->tag_list);
 
 	return 0;
 
-out_free_mq_map:
+free_mq_map:
 	kfree(set->mq_map);
 	set->mq_map = NULL;
-out_free_tags:
+free_sched_tags:
+	kfree(set->sched_tags);
+	set->sched_tags = NULL;
+free_drv_tags:
 	kfree(set->tags);
 	set->tags = NULL;
 	return ret;
@@ -2465,12 +2525,16 @@  void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
 		if (set->tags[i]) {
 			blk_mq_free_rqs(set, set->tags[i], i);
 			blk_mq_free_rq_map(set->tags[i]);
+			blk_mq_free_rq_map(set->sched_tags[i]);
 		}
 	}
 
 	kfree(set->mq_map);
 	set->mq_map = NULL;
 
+	kfree(set->sched_tags);
+	set->sched_tags = NULL;
+
 	kfree(set->tags);
 	set->tags = NULL;
 }
@@ -2482,14 +2546,18 @@  int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
 	struct blk_mq_hw_ctx *hctx;
 	int i, ret;
 
-	if (!set || nr > set->queue_depth)
+	if (!set)
 		return -EINVAL;
 
 	ret = 0;
 	queue_for_each_hw_ctx(q, hctx, i) {
 		if (!hctx->tags)
 			continue;
-		ret = blk_mq_tag_update_depth(hctx->tags, nr);
+		ret = blk_mq_tag_update_depth(hctx->tags,
+					      min(nr, set->queue_depth));
+		if (ret)
+			break;
+		ret = blk_mq_tag_update_depth(hctx->sched_tags, nr);
 		if (ret)
 			break;
 	}
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 2e98dd8ccee2..0368c513c2ab 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -31,6 +31,7 @@  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);
+int blk_mq_assign_drv_tag(struct request *rq);
 bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *, struct list_head *);
 void blk_mq_flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head *list);
 
@@ -41,7 +42,9 @@  void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 		     unsigned int hctx_idx);
 void blk_mq_free_rq_map(struct blk_mq_tags *tags);
 struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
-					unsigned int hctx_idx);
+					unsigned int hctx_idx,
+					unsigned int nr_tags,
+					unsigned int reserved_tags);
 int blk_mq_alloc_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 		     unsigned int hctx_idx);
 
diff --git a/block/blk-tag.c b/block/blk-tag.c
index bae1decb6ec3..319a3a3eb1d7 100644
--- a/block/blk-tag.c
+++ b/block/blk-tag.c
@@ -272,6 +272,7 @@  void blk_queue_end_tag(struct request_queue *q, struct request *rq)
 	list_del_init(&rq->queuelist);
 	rq->rq_flags &= ~RQF_QUEUED;
 	rq->tag = -1;
+	rq->sched_tag = -1;
 
 	if (unlikely(bqt->tag_index[tag] == NULL))
 		printk(KERN_ERR "%s: tag %d is missing\n",
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 9255ccb043f2..377594bcda8d 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -36,6 +36,7 @@  struct blk_mq_hw_ctx {
 	atomic_t		wait_index;
 
 	struct blk_mq_tags	*tags;
+	struct blk_mq_tags	*sched_tags;
 
 	struct srcu_struct	queue_rq_srcu;
 
@@ -72,6 +73,7 @@  struct blk_mq_tag_set {
 	void			*driver_data;
 
 	struct blk_mq_tags	**tags;
+	struct blk_mq_tags	**sched_tags;
 
 	struct mutex		tag_list_lock;
 	struct list_head	tag_list;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 7c40fb838b44..112b57bce9e9 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -223,6 +223,7 @@  struct request {
 	void *special;		/* opaque pointer available for LLD use */
 
 	int tag;
+	int sched_tag;
 	int errors;
 
 	/*
-- 
2.11.0