mq-deadline: the dd->dispatch queue follows a FIFO policy

In the initial implementation, the 'list_add(&rq->queuelist, ...' statement
added to the dd_insert_request function was designed to differentiate
priorities among various IO-requests within the same linked list. For
example, 'Commit 945ffb6 ("mq-deadline: add blk-mq adaptation of the
deadline IO scheduler")', introduced this 'list_add' operation to ensure
that requests with the at_head flag would always be dispatched before
requests without the REQ_TYPE_FS flag.

Since 'Commit 7687b38 ("bfq/mq-deadline: remove redundant check for
passthrough request")', removed blk_rq_is_passthrough, the dd->dispatch
list now contains only requests with the at_head flag. In this context,
all at_head requests should be treated as having equal priority, and a
first-in-first-out (FIFO) policy better aligns with the current situation.
Therefore, replacing list_add with list_add_tail is more appropriate.

Signed-off-by: Chengkaitao <chengkaitao@kylinos.cn>

Author: Chengkaitao

block/mq-deadline.c

@@ -661,7 +661,7 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
 	trace_block_rq_insert(rq);
 
 	if (flags & BLK_MQ_INSERT_AT_HEAD) {
-		list_add(&rq->queuelist, &dd->dispatch);
+		list_add_tail(&rq->queuelist, &dd->dispatch);
 		rq->fifo_time = jiffies;
 	} else {
 		deadline_add_rq_rb(per_prio, rq);