Commit 5b205071 authored Mar 11, 2022 by Jens Axboe

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>

parent 5ca7546f

block/blk-merge.c

+14 −6

Original line number	Diff line number	Diff line
		@@ -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);
		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;
		}