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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: d38a9c04c0 ("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>
This commit is contained in:
Jens Axboe 2022-03-11 10:21:43 -07:00
parent 5ca7546fe3
commit 5b2050718d
1 changed files with 14 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

20
block/blk-merge.c
View File

@ -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;
}