[PATCH 14/14] blk-mq-sched: improve IO scheduling on SCSI devcie

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

SCSI device often has per-request_queue queue depth
(.cmd_per_lun), which is applied among all hw queues
actually, and this patchset calls this as shared
queue depth.

One theory of scheduler is that we shouldn't dequeue
request from sw/scheduler queue and dispatch it to
driver when the low level queue is busy.

For SCSI device, queue being busy depends on the
per-request_queue limit, so we should hold all
hw queues if the request queue is busy.

This patch introduces per-request_queue dispatch
list for this purpose, and only when all requests
in this list are dispatched out successfully, we
can restart to dequeue request from sw/scheduler
queue and dispath it to lld.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq.c         |  8 +++++++-
 block/blk-mq.h         | 14 +++++++++++---
 include/linux/blkdev.h |  5 +++++
 3 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9b8b3a740d18..6d02901d798e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2667,8 +2667,14 @@ int blk_mq_update_sched_queue_depth(struct request_queue *q)
 	 * this queue depth limit
 	 */
 	if (q->queue_depth) {
-		queue_for_each_hw_ctx(q, hctx, i)
+		queue_for_each_hw_ctx(q, hctx, i) {
 			hctx->flags |= BLK_MQ_F_SHARED_DEPTH;
+			hctx->dispatch_lock = &q->__mq_dispatch_lock;
+			hctx->dispatch_list = &q->__mq_dispatch_list;
+
+			spin_lock_init(hctx->dispatch_lock);
+			INIT_LIST_HEAD(hctx->dispatch_list);
+		}
 	}
 
 	if (!q->elevator)
diff --git a/block/blk-mq.h b/block/blk-mq.h
index a8788058da56..4853d422836f 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -138,19 +138,27 @@ static inline bool blk_mq_hw_queue_mapped(struct blk_mq_hw_ctx *hctx)
 static inline bool blk_mq_hctx_is_busy(struct request_queue *q,
 		struct blk_mq_hw_ctx *hctx)
 {
-	return test_bit(BLK_MQ_S_BUSY, &hctx->state);
+	if (!(hctx->flags & BLK_MQ_F_SHARED_DEPTH))
+		return test_bit(BLK_MQ_S_BUSY, &hctx->state);
+	return q->mq_dispatch_busy;
 }
 
 static inline void blk_mq_hctx_set_busy(struct request_queue *q,
 		struct blk_mq_hw_ctx *hctx)
 {
-	set_bit(BLK_MQ_S_BUSY, &hctx->state);
+	if (!(hctx->flags & BLK_MQ_F_SHARED_DEPTH))
+		set_bit(BLK_MQ_S_BUSY, &hctx->state);
+	else
+		q->mq_dispatch_busy = 1;
 }
 
 static inline void blk_mq_hctx_clear_busy(struct request_queue *q,
 		struct blk_mq_hw_ctx *hctx)
 {
-	clear_bit(BLK_MQ_S_BUSY, &hctx->state);
+	if (!(hctx->flags & BLK_MQ_F_SHARED_DEPTH))
+		clear_bit(BLK_MQ_S_BUSY, &hctx->state);
+	else
+		q->mq_dispatch_busy = 0;
 }
 
 static inline bool blk_mq_has_dispatch_rqs(struct blk_mq_hw_ctx *hctx)
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 25f6a0cb27d3..bc0e607710f2 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -395,6 +395,11 @@ struct request_queue {
 
 	atomic_t		shared_hctx_restart;
 
+	/* blk-mq dispatch list and lock for shared queue depth case */
+	struct list_head	__mq_dispatch_list;
+	spinlock_t		__mq_dispatch_lock;
+	unsigned int		mq_dispatch_busy;
+
 	struct blk_queue_stats	*stats;
 	struct rq_wb		*rq_wb;
 
-- 
2.9.4