nvme-pci: reverse request order in nvme_queue_rqs

blk_mq_flush_plug_list submits requests in the reverse order that they
were submitted, which leads to a rather suboptimal I/O pattern especially
in rotational devices.  Fix this by rewriting nvme_queue_rqs so that it
always pops the requests from the passed in request list, and then adds
them to the head of a local submit list.  This actually simplifies the
code a bit as it removes the complicated list splicing, at the cost of
extra updates of the rq_next pointer.  As that should be cache hot
anyway it should be an easy price to pay.

Fixes: d62cbcf ("nvme: add support for mq_ops->queue_rqs()")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20241113152050.157179-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/nvme/host/pci.c

@@ -906,9 +906,10 @@ static blk_status_t nvme_queue_rq(struct blk_mq_hw_ctx *hctx,
 
 static void nvme_submit_cmds(struct nvme_queue *nvmeq, struct request **rqlist)
 {
+	struct request *req;
+
 	spin_lock(&nvmeq->sq_lock);
-	while (!rq_list_empty(*rqlist)) {
-		struct request *req = rq_list_pop(rqlist);
+	while ((req = rq_list_pop(rqlist))) {
 		struct nvme_iod *iod = blk_mq_rq_to_pdu(req);
 
 		nvme_sq_copy_cmd(nvmeq, &iod->cmd);
@@ -933,31 +934,25 @@ static bool nvme_prep_rq_batch(struct nvme_queue *nvmeq, struct request *req)
 
 static void nvme_queue_rqs(struct request **rqlist)
 {
-	struct request *req, *next, *prev = NULL;
+	struct request *submit_list = NULL;
 	struct request *requeue_list = NULL;
+	struct request **requeue_lastp = &requeue_list;
+	struct nvme_queue *nvmeq = NULL;
+	struct request *req;
 
-	rq_list_for_each_safe(rqlist, req, next) {
-		struct nvme_queue *nvmeq = req->mq_hctx->driver_data;
-
-		if (!nvme_prep_rq_batch(nvmeq, req)) {
-			/* detach 'req' and add to remainder list */
-			rq_list_move(rqlist, &requeue_list, req, prev);
-
-			req = prev;
-			if (!req)
-				continue;
-		}
+	while ((req = rq_list_pop(rqlist))) {
+		if (nvmeq && nvmeq != req->mq_hctx->driver_data)
+			nvme_submit_cmds(nvmeq, &submit_list);
+		nvmeq = req->mq_hctx->driver_data;
 
-		if (!next || req->mq_hctx != next->mq_hctx) {
-			/* detach rest of list, and submit */
-			req->rq_next = NULL;
-			nvme_submit_cmds(nvmeq, rqlist);
-			*rqlist = next;
-			prev = NULL;
-		} else
-			prev = req;
+		if (nvme_prep_rq_batch(nvmeq, req))
+			rq_list_add(&submit_list, req); /* reverse order */
+		else
+			rq_list_add_tail(&requeue_lastp, req);
 	}
 
+	if (nvmeq)
+		nvme_submit_cmds(nvmeq, &submit_list);
 	*rqlist = requeue_list;
 }