[PATCH] block: elevator.c: Check elevator kernel argument again

[Marcos Paulo de Souza]

Since the inclusion of blk-mq, elevator= kernel argument was not being
considered anymore, making it impossible to specify a specific elevator
at boot time as it was used before.

This is done by checking chosen_elevator global variable, which is
populated once elevator= kernel argument is passed. Without this patch,
mq-deadline is the only elevator that is can be used at boot time.

Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@xxxxxxxxx>
---

 I found this issue while inspecting why noop scheduler was gone, and so I found
 that was now impossible to use a scheduler different from mq-deadeline.

 Am I missing something? Is this a desirable behavior?

 One more question: currently we can't specify a "none" scheduler, like it used
 to be "noop". This is also on purpose? If it's not, I can provide a patch for
 it.

 block/elevator.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
index 2f17d66d0e61..41ce7ba099ba 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -601,7 +601,7 @@ int elevator_switch_mq(struct request_queue *q,
  */
 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)
@@ -615,9 +615,18 @@ 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 elevator was used as kernel argument, try to load it */
+	if (*chosen_elevator) {
+		e = elevator_get(q, chosen_elevator, false);
+		if (!e)
+			pr_err("io scheduler %s not found", chosen_elevator);
+	}
+
+	if (!e) {
+		e = elevator_get(q, "mq-deadline", false);
+		if (!e)
+			goto out_unlock;
+	}
 
 	err = blk_mq_init_sched(q, e);
 	if (err)
-- 
2.22.0