block: ensure plug merging checks the correct queue at least once

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: d38a9c04 ("block: only check previous entry for plug merge attempt")
Reported-and-tested-by: Song Liu <song@kernel.org>
Reviewed-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-merge.c

@@ -1087,12 +1087,20 @@ bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
 	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) {
-		if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
-				BIO_MERGE_OK)
-			return true;
+	rq_list_for_each(&plug->mq_list, rq) {
+		if (rq->q == q) {
+			if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
+			    BIO_MERGE_OK)
+				return true;
+			break;
+		}
+
+		/*
+		 * Only keep iterating plug list for merges if we have multiple
+		 * queues
+		 */
+		if (!plug->multiple_queues)
+			break;
 	}
 	return false;
 }