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

Ming Lei
2017-07-31T16:53:41Z

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