[PATCH 5.17 0012/1126] block: ensure plug merging checks the correct queue at least once

Greg Kroah-Hartman
2022-04-05T07:46:49Z

From: Jens Axboe <axboe@kernel.dk>

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@kernel.org>
Reviewed-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
block/blk-merge.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)

--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -1089,12 +1089,20 @@ 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) {
- 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;
}