block: avoid to quiesce queue in elevator_init_mq

elevator_init_mq() is only called before adding disk, when there isn't
any FS I/O, only passthrough requests can be queued, so freezing queue
plus canceling dispatch work is enough to drain any dispatch activities,
then we can avoid synchronize_srcu() in blk_mq_quiesce_queue().

Long boot latency issue can be fixed in case of lots of disks added
during booting.

Fixes: 737eb78e82 ("block: Delay default elevator initialization")
Reported-by: yangerkun <yangerkun@huawei.com>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20211117115502.1600950-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/elevator.c

@@ -694,12 +694,18 @@ void elevator_init_mq(struct request_queue *q)
 	if (!e)
 		return;
 
+	/*
+	 * We are called before adding disk, when there isn't any FS I/O,
+	 * so freezing queue plus canceling dispatch work is enough to
+	 * drain any dispatch activities originated from passthrough
+	 * requests, then no need to quiesce queue which may add long boot
+	 * latency, especially when lots of disks are involved.
+	 */
 	blk_mq_freeze_queue(q);
-	blk_mq_quiesce_queue(q);
+	blk_mq_cancel_work_sync(q);
 
 	err = blk_mq_init_sched(q, e);
 
-	blk_mq_unquiesce_queue(q);
 	blk_mq_unfreeze_queue(q);
 
 	if (err) {