[PATCH] block: BFQ default for single queue devices

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

This sets BFQ as the default scheduler for single queue
block devices (nr_hw_queues == 1) if it is available. This
affects notably MMC/SD-cards but notably also UBI and
the loopback device.

I have been running it for a while without any negative
effects on my pet systems and I want some wider testing
so let's throw it out there and see what people say.
Admittedly my use cases are limited.

I talked to Pavel a bit back and it turns out he has a
usecase for BFQ as well and I bet he also would like it
as default scheduler for that system (Pavel tell us more,
I don't remember what it was!)

Intuitively I could understand that maybe we want to
leave the loop device (possibly others? nbd? rbd?) as
"none", as it is probably relying on a scheduler on the
device below it, so I'm open to passing in a scheduler hint
from the respective subsystem in say struct blk_mq_tag_set.
However that makes for a bit of syntactic dissonance
with the struct member ".nr_hw_queues" (I wonder how
the loop device can have 1 "hardware queue"?) so
maybe we should in that case also rename that struct
member to ".nr_queues" fair and square before we start
making adjustments for treating queues differently whether
they are in hardware or actually not.

Cc: Pavel Machek <pavel@xxxxxx>
Cc: Paolo Valente <paolo.valente@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Ulf Hansson <ulf.hansson@xxxxxxxxxx>
Cc: Richard Weinberger <richard@xxxxxx>
Cc: Artem Bityutskiy <dedekind1@xxxxxxxxx>
Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
---
 block/elevator.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
index e18ac68626e3..e5a2c39eee7b 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -948,13 +948,15 @@ int elevator_switch_mq(struct request_queue *q,
 }
 
 /*
- * For blk-mq devices, we default to using mq-deadline, if available, for single
- * queue devices.  If deadline isn't available OR we have multiple queues,
- * default to "none".
+ * For blk-mq devices, we default to using:
+ * - "none" for multiqueue devices (nr_hw_queues != 1)
+ * - "bfq", if available, for single queue devices
+ * - "mq-deadline" if "bfq" is not available for single queue devices
+ * - "none" for single queue devices as well as last resort
  */
 int elevator_init_mq(struct request_queue *q)
 {
-	struct elevator_type *e;
+	struct elevator_type *e = NULL;
 	int err = 0;
 
 	if (q->nr_hw_queues != 1)
@@ -968,9 +970,14 @@ int elevator_init_mq(struct request_queue *q)
 	if (unlikely(q->elevator))
 		goto out_unlock;
 
-	e = elevator_get(q, "mq-deadline", false);
-	if (!e)
-		goto out_unlock;
+	if (IS_ENABLED(CONFIG_IOSCHED_BFQ))
+		e = elevator_get(q, "bfq", false);
+
+	if (!e) {
+		e = elevator_get(q, "mq-deadline", false);
+		if (!e)
+			goto out_unlock;
+	}
 
 	err = blk_mq_init_sched(q, e);
 	if (err)
-- 
2.17.1


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