block: don't reorder requests in blk_add_rq_to_plug

Add requests to the tail of the list instead of the front so that they
are queued up in submission order.

Remove the re-reordering in blk_mq_dispatch_plug_list, virtio_queue_rqs
and nvme_queue_rqs now that the list is ordered as expected.

Signed-off-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Christoph Hellwig

block/blk-mq.c

@@ -1392,7 +1392,7 @@ static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
 	 */
 	if (!plug->has_elevator && (rq->rq_flags & RQF_SCHED_TAGS))
 		plug->has_elevator = true;
-	rq_list_add_head(&plug->mq_list, rq);
+	rq_list_add_tail(&plug->mq_list, rq);
 	plug->rq_count++;
 }
 
@@ -2844,7 +2844,7 @@ static void blk_mq_dispatch_plug_list(struct blk_plug *plug, bool from_sched)
 			rq_list_add_tail(&requeue_list, rq);
 			continue;
 		}
-		list_add(&rq->queuelist, &list);
+		list_add_tail(&rq->queuelist, &list);
 		depth++;
 	} while (!rq_list_empty(&plug->mq_list));
 

drivers/block/virtio_blk.c

@@ -514,7 +514,7 @@ static void virtio_queue_rqs(struct rq_list *rqlist)
 		vq = this_vq;
 
 		if (virtblk_prep_rq_batch(req))
-			rq_list_add_head(&submit_list, req); /* reverse order */
+			rq_list_add_tail(&submit_list, req);
 		else
 			rq_list_add_tail(&requeue_list, req);
 	}

drivers/nvme/host/pci.c

@@ -945,7 +945,7 @@ static void nvme_queue_rqs(struct rq_list *rqlist)
 		nvmeq = req->mq_hctx->driver_data;
 
 		if (nvme_prep_rq_batch(nvmeq, req))
-			rq_list_add_head(&submit_list, req); /* reverse order */
+			rq_list_add_tail(&submit_list, req);
 		else
 			rq_list_add_tail(&requeue_list, req);
 	}