[PATCH v4 3/7] block/mq-deadline: Only use zone locking if necessary

[Bart Van Assche <bvanassche@xxxxxxx>]

Measurements have shown that limiting the queue depth to one for zoned
writes has a significant negative performance impact on zoned UFS devices.
Hence this patch that disables zone locking by the mq-deadline scheduler
if zoned writes are submitted in order and if the storage controller
preserves the command order. This patch is based on the following
assumptions:
- It happens infrequently that zoned write requests are reordered by the
  block layer.
- The I/O priority of all write requests is the same per zone.
- Either no I/O scheduler is used or an I/O scheduler is used that
  submits write requests per zone in LBA order.

Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Damien Le Moal <dlemoal@xxxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 block/mq-deadline.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 02a916ba62ee..9a64577fe942 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -338,6 +338,17 @@ static struct request *deadline_skip_seq_writes(struct deadline_data *dd,
 	return rq;
 }
 
+/*
+ * Use write locking if either QUEUE_FLAG_NO_ZONE_WRITE_LOCK or
+ * REQ_NO_ZONE_WRITE_LOCK has not been set. Not using zone write locking is
+ * only safe if the submitter allocates and submit requests in LBA order per
+ * zone and if the block driver preserves the request order.
+ */
+static bool dd_use_write_locking(struct request *rq)
+{
+	return blk_queue_is_zoned(rq->q) && !blk_no_zone_write_lock(rq);
+}
+
 /*
  * For the specified data direction, return the next request to
  * dispatch using arrival ordered lists.
@@ -353,7 +364,7 @@ deadline_fifo_request(struct deadline_data *dd, struct dd_per_prio *per_prio,
 		return NULL;
 
 	rq = rq_entry_fifo(per_prio->fifo_list[data_dir].next);
-	if (data_dir == DD_READ || !blk_queue_is_zoned(rq->q))
+	if (data_dir == DD_READ || !dd_use_write_locking(rq))
 		return rq;
 
 	/*
@@ -398,7 +409,7 @@ deadline_next_request(struct deadline_data *dd, struct dd_per_prio *per_prio,
 	if (!rq)
 		return NULL;
 
-	if (data_dir == DD_READ || !blk_queue_is_zoned(rq->q))
+	if (data_dir == DD_READ || !dd_use_write_locking(rq))
 		return rq;
 
 	/*
@@ -526,8 +537,9 @@ static struct request *__dd_dispatch_request(struct deadline_data *dd,
 	}
 
 	/*
-	 * For a zoned block device, if we only have writes queued and none of
-	 * them can be dispatched, rq will be NULL.
+	 * For a zoned block device that requires write serialization, if we
+	 * only have writes queued and none of them can be dispatched, rq will
+	 * be NULL.
 	 */
 	if (!rq)
 		return NULL;
@@ -552,7 +564,8 @@ static struct request *__dd_dispatch_request(struct deadline_data *dd,
 	/*
 	 * If the request needs its target zone locked, do it.
 	 */
-	blk_req_zone_write_lock(rq);
+	if (dd_use_write_locking(rq))
+		blk_req_zone_write_lock(rq);
 	rq->rq_flags |= RQF_STARTED;
 	return rq;
 }
@@ -933,7 +946,7 @@ static void dd_finish_request(struct request *rq)
 
 	atomic_inc(&per_prio->stats.completed);
 
-	if (blk_queue_is_zoned(q)) {
+	if (dd_use_write_locking(rq)) {
 		unsigned long flags;
 
 		spin_lock_irqsave(&dd->zone_lock, flags);