[PATCH] block: Flag elevators suitable for single queue

[Linus Walleij <linus.walleij@linaro.org>]

The Kyber block scheduler is not suitable for single hardware
queue devices, so add a new flag for single hardware queue
devices and add that to the deadline and BFQ schedulers
so the Kyber scheduler will not be selected for single queue
devices.

Deadline and BFQ are applicable to single HW queues so flag
each of these as single HW queue-friendly.

Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 block/bfq-iosched.c      | 1 +
 block/elevator.c         | 3 +++
 block/mq-deadline.c      | 3 ++-
 include/linux/elevator.h | 2 ++
 4 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 3d411716d7ee..7bf99fd83472 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -6812,6 +6812,7 @@ static struct elevator_type iosched_bfq_mq = {
 	.icq_align =		__alignof__(struct bfq_io_cq),
 	.elevator_attrs =	bfq_attrs,
 	.elevator_name =	"bfq",
+	.elevator_features =	ELEVATOR_F_SINGLE_HW_QUEUE,
 	.elevator_owner =	THIS_MODULE,
 };
 MODULE_ALIAS("bfq-iosched");
diff --git a/block/elevator.c b/block/elevator.c
index 4eab3d70e880..ebb4fc875b86 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -678,6 +678,9 @@ void elevator_init_mq(struct request_queue *q)
 	if (unlikely(q->elevator))
 		return;
 
+	if (q->nr_hw_queues == 1)
+		q->required_elevator_features |= ELEVATOR_F_SINGLE_HW_QUEUE;
+
 	if (!q->required_elevator_features)
 		e = elevator_get_default(q);
 	else
diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index b490f47fd553..324047add271 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -794,7 +794,8 @@ static struct elevator_type mq_deadline = {
 	.elevator_attrs = deadline_attrs,
 	.elevator_name = "mq-deadline",
 	.elevator_alias = "deadline",
-	.elevator_features = ELEVATOR_F_ZBD_SEQ_WRITE,
+	.elevator_features = ELEVATOR_F_ZBD_SEQ_WRITE |
+	ELEVATOR_F_SINGLE_HW_QUEUE,
 	.elevator_owner = THIS_MODULE,
 };
 MODULE_ALIAS("mq-deadline-iosched");
diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index 901bda352dcb..03057fa2f569 100644
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -172,6 +172,8 @@ extern struct request *elv_rb_find(struct rb_root *, sector_t);
 
 /* Supports zoned block devices sequential write constraint */
 #define ELEVATOR_F_ZBD_SEQ_WRITE	(1U << 0)
+/* Elevator is suitable for single hardware queue devices */
+#define ELEVATOR_F_SINGLE_HW_QUEUE	(1U << 1)
 
 #endif /* CONFIG_BLOCK */
 #endif
-- 
2.25.4