blk-mq: set default elevator as deadline in case of hctx shared tagset

Yanhui found that write performance is degraded a lot after applying
hctx shared tagset on one test machine with megaraid_sas. And turns out
it is caused by none scheduler which becomes default elevator caused by
hctx shared tagset patchset.

Given more scsi HBAs will apply hctx shared tagset, and the similar
performance exists for them too.

So keep previous behavior by still using default mq-deadline for queues
which apply hctx shared tagset, just like before.

Fixes: 32bc15a ("blk-mq: Facilitate a shared sbitmap per tagset")
Reported-by: Yanhui Ma <[email protected]>
Cc: John Garry <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Reviewed-by: Martin K. Petersen <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Reviewed-by: John Garry <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

block/elevator.c

@@ -621,7 +621,8 @@ static inline bool elv_support_iosched(struct request_queue *q)
  */
 static struct elevator_type *elevator_get_default(struct request_queue *q)
 {
-	if (q->nr_hw_queues != 1)
+	if (q->nr_hw_queues != 1 &&
+			!blk_mq_is_sbitmap_shared(q->tag_set->flags))
 		return NULL;
 
 	return elevator_get(q, "mq-deadline", false);