[PATCH 5.16 0030/1017] block: ensure plug merging checks the correct queue at least once

[Greg Kroah-Hartman]

From: Jens Axboe <axboe@xxxxxxxxx>

commit 5b2050718d095cd3242d1f42aaaea3a2fec8e6f0 upstream.

Song reports that a RAID rebuild workload runs much slower recently,
and it is seeing a lot less merging than it did previously. The reason
is that a previous commit reduced the amount of work we do for plug
merging. RAID rebuild interleaves requests between disks, so a last-entry
check in plug merging always misses a merge opportunity since we always
find a different disk than what we are looking for.

Modify the logic such that it's still a one-hit cache, but ensure that
we check enough to find the right target before giving up.

Fixes: d38a9c04c0d5 ("block: only check previous entry for plug merge attempt")
Reported-and-tested-by: Song Liu <song@xxxxxxxxxx>
Reviewed-by: Song Liu <songliubraving@xxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 block/blk-merge.c |   23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -1093,18 +1093,21 @@ bool blk_attempt_plug_merge(struct reque
 	if (!plug || rq_list_empty(plug->mq_list))
 		return false;
 
-	/* check the previously added entry for a quick merge attempt */
-	rq = rq_list_peek(&plug->mq_list);
-	if (rq->q == q) {
+	rq_list_for_each(&plug->mq_list, rq) {
+		if (rq->q == q) {
+			*same_queue_rq = true;
+			if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
+			    BIO_MERGE_OK)
+				return true;
+			break;
+		}
+
 		/*
-		 * Only blk-mq multiple hardware queues case checks the rq in
-		 * the same queue, there should be only one such rq in a queue
+		 * Only keep iterating plug list for merges if we have multiple
+		 * queues
 		 */
-		*same_queue_rq = true;
-
-		if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
-				BIO_MERGE_OK)
-			return true;
+		if (!plug->multiple_queues)
+			break;
 	}
 	return false;
 }