[PATCH 2/4] block/mq-deadline: serialize request dispatching

[Jens Axboe <axboe@xxxxxxxxx>]

If we're entering request dispatch but someone else is already
dispatching, then just skip this dispatch. We know IO is inflight and
this will trigger another dispatch event for any completion. This will
potentially cause slightly lower queue depth for contended cases, but
those are slowed down anyway and this should not cause an issue.

By itself, this patch doesn't help a whole lot, as the dispatch
lock contention reduction is just eating up by the same dd->lock now
seeing increased insertion contention. But it's required work to be
able to reduce the lock contention in general.

Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
 block/mq-deadline.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 9b7563e9d638..b579ce282176 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -79,10 +79,20 @@ struct dd_per_prio {
 	struct io_stats_per_prio stats;
 };
 
+enum {
+	DD_DISPATCHING	= 0,
+};
+
 struct deadline_data {
 	/*
 	 * run time data
 	 */
+	struct {
+		spinlock_t lock;
+		spinlock_t zone_lock;
+	} ____cacheline_aligned_in_smp;
+
+	unsigned long run_state;
 
 	struct dd_per_prio per_prio[DD_PRIO_COUNT];
 
@@ -100,9 +110,6 @@ struct deadline_data {
 	int front_merges;
 	u32 async_depth;
 	int prio_aging_expire;
-
-	spinlock_t lock;
-	spinlock_t zone_lock;
 };
 
 /* Maps an I/O priority class to a deadline scheduler priority. */
@@ -600,6 +607,18 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
 	struct request *rq;
 	enum dd_prio prio;
 
+	/*
+	 * If someone else is already dispatching, skip this one. This will
+	 * defer the next dispatch event to when something completes, and could
+	 * potentially lower the queue depth for contended cases.
+	 *
+	 * See the logic in blk_mq_do_dispatch_sched(), which loops and
+	 * retries if nothing is dispatched.
+	 */
+	if (test_bit(DD_DISPATCHING, &dd->run_state) ||
+	    test_and_set_bit(DD_DISPATCHING, &dd->run_state))
+		return NULL;
+
 	spin_lock(&dd->lock);
 	rq = dd_dispatch_prio_aged_requests(dd, now);
 	if (rq)
@@ -616,6 +635,7 @@ static struct request *dd_dispatch_request(struct blk_mq_hw_ctx *hctx)
 	}
 
 unlock:
+	clear_bit(DD_DISPATCHING, &dd->run_state);
 	spin_unlock(&dd->lock);
 
 	return rq;
@@ -706,6 +726,9 @@ static int dd_init_sched(struct request_queue *q, struct elevator_type *e)
 
 	eq->elevator_data = dd;
 
+	spin_lock_init(&dd->lock);
+	spin_lock_init(&dd->zone_lock);
+
 	for (prio = 0; prio <= DD_PRIO_MAX; prio++) {
 		struct dd_per_prio *per_prio = &dd->per_prio[prio];
 
@@ -722,8 +745,6 @@ static int dd_init_sched(struct request_queue *q, struct elevator_type *e)
 	dd->last_dir = DD_WRITE;
 	dd->fifo_batch = fifo_batch;
 	dd->prio_aging_expire = prio_aging_expire;
-	spin_lock_init(&dd->lock);
-	spin_lock_init(&dd->zone_lock);
 
 	/* We dispatch from request queue wide instead of hw queue */
 	blk_queue_flag_set(QUEUE_FLAG_SQ_SCHED, q);
-- 
2.43.0