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; }