On 11/27/18 4:31 PM, Omar Sandoval wrote:

On Mon, Nov 26, 2018 at 09:35:50AM -0700, Jens Axboe wrote:

quoted

Do it for the nr_hw_queues == 1 case, but only do it for the multi queue
case if we have requests for multiple devices in the plug.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/blk-core.c       | 1 +
 block/blk-mq.c         | 7 +++++--
 include/linux/blkdev.h | 1 +
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index be9233400314..c9758d185357 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -1780,6 +1780,7 @@ void blk_start_plug(struct blk_plug *plug)
 	INIT_LIST_HEAD(&plug->mq_list);
 	INIT_LIST_HEAD(&plug->cb_list);
 	plug->rq_count = 0;
+	plug->do_sort = false;
 
 	/*
 	 * Store ordering should not be needed here, since a potential

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 99c66823d52f..6a249bf6ed00 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -1678,7 +1678,8 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 	list_splice_init(&plug->mq_list, &list);
 	plug->rq_count = 0;
 
-	list_sort(NULL, &list, plug_rq_cmp);
+	if (plug->do_sort)
+		list_sort(NULL, &list, plug_rq_cmp);
 
 	this_q = NULL;
 	this_hctx = NULL;

@@ -1935,6 +1936,7 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 
 		list_add_tail(&rq->queuelist, &plug->mq_list);
 		plug->rq_count++;
+		plug->do_sort = true;
 	} else if (plug && !blk_queue_nomerges(q)) {
 		blk_mq_bio_to_request(rq, bio);
 

@@ -1958,7 +1960,8 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 			data.hctx = same_queue_rq->mq_hctx;
 			blk_mq_try_issue_directly(data.hctx, same_queue_rq,
 					&cookie);
-		}
+		} else if (plug->rq_count > 1)
+			plug->do_sort = true;

If plug->rq_count == 2, there's no benefit to sorting, either. The
nr_hw_queues == 1 case could also avoid sorting in that case. So maybe
this whole patch could just be replaced with:

Heh yes, good point, it should be 3 at least. But if you look at the
later mq plug patch, we only sort for that one if we have multiple
queues. So the logic should be something ala:

if (plug->rq_count > 2 && plug->has_multiple_queues)

since that's the only case we want to sort for.

-- 
Jens Axboe