cfq-iosched: fix the setting of IOPS mode on SSDs

A previous commit wanted to make CFQ default to IOPS mode on
non-rotational storage, however it did so when the queue was
initialized and the non-rotational flag is only set later on
in the probe.

Add an elevator hook that gets called off the add_disk() path,
at that point we know that feature probing has finished, and
we can reliably check for the various flags that drivers can
set.

Fixes: 41c0126b ("block: Make CFQ default to IOPS mode on SSDs")
Tested-by: Romain Francoise <romain@orebokech.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/cfq-iosched.c

@@ -4508,7 +4508,7 @@ static int cfq_init_queue(struct request_queue *q, struct elevator_type *e)
 	cfqd->cfq_slice[1] = cfq_slice_sync;
 	cfqd->cfq_target_latency = cfq_target_latency;
 	cfqd->cfq_slice_async_rq = cfq_slice_async_rq;
-	cfqd->cfq_slice_idle = blk_queue_nonrot(q) ? 0 : cfq_slice_idle;
+	cfqd->cfq_slice_idle = cfq_slice_idle;
 	cfqd->cfq_group_idle = cfq_group_idle;
 	cfqd->cfq_latency = 1;
 	cfqd->hw_tag = -1;
@@ -4525,6 +4525,18 @@ out_free:
 	return ret;
 }
 
+static void cfq_registered_queue(struct request_queue *q)
+{
+	struct elevator_queue *e = q->elevator;
+	struct cfq_data *cfqd = e->elevator_data;
+
+	/*
+	 * Default to IOPS mode with no idling for SSDs
+	 */
+	if (blk_queue_nonrot(q))
+		cfqd->cfq_slice_idle = 0;
+}
+
 /*
  * sysfs parts below -->
  */
@@ -4640,6 +4652,7 @@ static struct elevator_type iosched_cfq = {
 		.elevator_may_queue_fn =	cfq_may_queue,
 		.elevator_init_fn =		cfq_init_queue,
 		.elevator_exit_fn =		cfq_exit_queue,
+		.elevator_registered_fn =	cfq_registered_queue,
 	},
 	.icq_size	=	sizeof(struct cfq_io_cq),
 	.icq_align	=	__alignof__(struct cfq_io_cq),

block/elevator.c

@@ -810,6 +810,8 @@ int elv_register_queue(struct request_queue *q)
 		}
 		kobject_uevent(&e->kobj, KOBJ_ADD);
 		e->registered = 1;
+		if (e->type->ops.elevator_registered_fn)
+			e->type->ops.elevator_registered_fn(q);
 	}
 	return error;
 }

include/linux/elevator.h

@@ -39,6 +39,7 @@ typedef void (elevator_deactivate_req_fn) (struct request_queue *, struct reques
 typedef int (elevator_init_fn) (struct request_queue *,
 				struct elevator_type *e);
 typedef void (elevator_exit_fn) (struct elevator_queue *);
+typedef void (elevator_registered_fn) (struct request_queue *);
 
 struct elevator_ops
 {
@@ -68,6 +69,7 @@ struct elevator_ops
 
 	elevator_init_fn *elevator_init_fn;
 	elevator_exit_fn *elevator_exit_fn;
+	elevator_registered_fn *elevator_registered_fn;
 };
 
 #define ELV_NAME_MAX	(16)