block: mq-deadline: Do not break sequential write streams to zoned HDDs

mq-deadline ensures an in order dispatching of write requests to zoned
block devices using a per zone lock (a bit). This implies that for any
purely sequential write workload, the drive is exercised most of the
time at a maximum queue depth of one.

However, when such sequential write workload crosses a zone boundary
(when sequentially writing multiple contiguous zones), zone write
locking may prevent the last write to one zone to be issued (as the
previous write is still being executed) but allow the first write to the
following zone to be issued (as that zone is not yet being writen and
not locked). This result in an out of order delivery of the sequential
write commands to the device every time a zone boundary is crossed.

While such behavior does not break the sequential write constraint of
zoned block devices (and does not generate any write error), some zoned
hard-disks react badly to seeing these out of order writes, resulting in
lower write throughput.

This problem can be addressed by always dispatching the first request
of a stream of sequential write requests, regardless of the zones
targeted by these sequential writes. To do so, the function
deadline_skip_seq_writes() is introduced and used in
deadline_next_request() to select the next write command to issue if the
target device is an HDD (blk_queue_nonrot() being false).
deadline_fifo_request() is modified using the new
deadline_earlier_request() and deadline_is_seq_write() helpers to ignore
requests in the fifo list that have a preceding request in lba order
that is sequential.

With this fix, a sequential write workload executed with the following
fio command:

fio  --name=seq-write --filename=/dev/sda --zonemode=zbd --direct=1 \
     --size=68719476736  --ioengine=libaio --iodepth=32 --rw=write \
     --bs=65536

results in an increase from 225 MB/s to 250 MB/s of the write throughput
of an SMR HDD (11% increase).

Cc: <stable@vger.kernel.org>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20221124021208.242541-3-damien.lemoal@opensource.wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 62 insertions, 4 deletions

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 36374481cb87..6672f1bce379 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -131,6 +131,20 @@ static u8 dd_rq_ioclass(struct request *rq)
 }
 
 /*
+ * get the request before `rq' in sector-sorted order
+ */
+static inline struct request *
+deadline_earlier_request(struct request *rq)
+{
+	struct rb_node *node = rb_prev(&rq->rb_node);
+
+	if (node)
+		return rb_entry_rq(node);
+
+	return NULL;
+}
+
+/*
  * get the request after `rq' in sector-sorted order
  */
 static inline struct request *
@@ -278,6 +292,39 @@ static inline int deadline_check_fifo(struct dd_per_prio *per_prio,
 }
 
 /*
+ * Check if rq has a sequential request preceding it.
+ */
+static bool deadline_is_seq_writes(struct deadline_data *dd, struct request *rq)
+{
+	struct request *prev = deadline_earlier_request(rq);
+
+	if (!prev)
+		return false;
+
+	return blk_rq_pos(prev) + blk_rq_sectors(prev) == blk_rq_pos(rq);
+}
+
+/*
+ * Skip all write requests that are sequential from @rq, even if we cross
+ * a zone boundary.
+ */
+static struct request *deadline_skip_seq_writes(struct deadline_data *dd,
+						struct request *rq)
+{
+	sector_t pos = blk_rq_pos(rq);
+	sector_t skipped_sectors = 0;
+
+	while (rq) {
+		if (blk_rq_pos(rq) != pos + skipped_sectors)
+			break;
+		skipped_sectors += blk_rq_sectors(rq);
+		rq = deadline_latter_request(rq);
+	}
+
+	return rq;
+}
+
+/*
  * For the specified data direction, return the next request to
  * dispatch using arrival ordered lists.
  */
@@ -297,11 +344,16 @@ deadline_fifo_request(struct deadline_data *dd, struct dd_per_prio *per_prio,
 
 	/*
 	 * Look for a write request that can be dispatched, that is one with
-	 * an unlocked target zone.
+	 * an unlocked target zone. For some HDDs, breaking a sequential
+	 * write stream can lead to lower throughput, so make sure to preserve
+	 * sequential write streams, even if that stream crosses into the next
+	 * zones and these zones are unlocked.
 	 */
 	spin_lock_irqsave(&dd->zone_lock, flags);
 	list_for_each_entry(rq, &per_prio->fifo_list[DD_WRITE], queuelist) {
-		if (blk_req_can_dispatch_to_zone(rq))
+		if (blk_req_can_dispatch_to_zone(rq) &&
+		    (blk_queue_nonrot(rq->q) ||
+		     !deadline_is_seq_writes(dd, rq)))
 			goto out;
 	}
 	rq = NULL;
@@ -331,13 +383,19 @@ deadline_next_request(struct deadline_data *dd, struct dd_per_prio *per_prio,
 
 	/*
 	 * Look for a write request that can be dispatched, that is one with
-	 * an unlocked target zone.
+	 * an unlocked target zone. For some HDDs, breaking a sequential
+	 * write stream can lead to lower throughput, so make sure to preserve
+	 * sequential write streams, even if that stream crosses into the next
+	 * zones and these zones are unlocked.
 	 */
 	spin_lock_irqsave(&dd->zone_lock, flags);
 	while (rq) {
 		if (blk_req_can_dispatch_to_zone(rq))
 			break;
-		rq = deadline_latter_request(rq);
+		if (blk_queue_nonrot(rq->q))
+			rq = deadline_latter_request(rq);
+		else
+			rq = deadline_skip_seq_writes(dd, rq);
 	}
 	spin_unlock_irqrestore(&dd->zone_lock, flags);