It's expensive to browse the whole plug list for merge opportunities at
the IOPS rates that modern storage can do. For sequential IO, the one-hit
cached merge should suffice on fast drives, and for rotational storage the
IO scheduler will do a more exhaustive lookup based merge anyway.
Just remove the plug based O(N) traversal merging.
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 762da71f9fde..e10f49decd06 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -1043,62 +1043,6 @@ static enum bio_merge_status blk_attempt_bio_merge(struct request_queue *q,
return BIO_MERGE_FAILED;
}
-/**
- * blk_attempt_plug_merge - try to merge with %current's plugged list
- * @q: request_queue new bio is being queued at
- * @bio: new bio being queued
- * @nr_segs: number of segments in @bio
- * @same_queue_rq: pointer to &struct request that gets filled in when
- * another request associated with @q is found on the plug list
- * (optional, may be %NULL)
- *
- * Determine whether @bio being queued on @q can be merged with a request
- * on %current's plugged list. Returns %true if merge was successful,
- * otherwise %false.
- *
- * Plugging coalesces IOs from the same issuer for the same purpose without
- * going through @q->queue_lock. As such it's more of an issuing mechanism
- * than scheduling, and the request, while may have elvpriv data, is not
- * added on the elevator at this point. In addition, we don't have
- * reliable access to the elevator outside queue lock. Only check basic
- * merging parameters without querying the elevator.
- *
- * Caller must ensure !blk_queue_nomerges(q) beforehand.
- */
-bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
- unsigned int nr_segs, struct request **same_queue_rq)
-{
- struct blk_plug *plug;
- struct request *rq;
- struct list_head *plug_list;
-
- plug = blk_mq_plug(q, bio);
- if (!plug)
- return false;
-
- plug_list = &plug->mq_list;
-
- list_for_each_entry_reverse(rq, plug_list, queuelist) {
- if (rq->q == q && same_queue_rq) {
- /*
- * Only blk-mq multiple hardware queues case checks the
- * rq in the same queue, there should be only one such
- * rq in a queue
- **/
- *same_queue_rq = rq;
- }
-
- if (rq->q != q)
- continue;
-
- if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
- BIO_MERGE_OK)
- return true;
- }
-
- return false;
-}
-
/*
* Iterate list of requests and see if we can merge this bio with any
* of them.
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 5777064f1174..4c5b34787bbf 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2323,10 +2323,6 @@ void blk_mq_submit_bio(struct bio *bio)
if (!bio_integrity_prep(bio))
goto queue_exit;
- if (!is_flush_fua && !blk_queue_nomerges(q) &&
- blk_attempt_plug_merge(q, bio, nr_segs, &same_queue_rq))
- goto queue_exit;
-
if (blk_mq_sched_bio_merge(q, bio, nr_segs))
goto queue_exit;
diff --git a/block/blk.h b/block/blk.h
index fa23338449ed..0afee3e6a7c1 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -214,8 +214,6 @@ static inline void blk_integrity_del(struct gendisk *disk)
unsigned long blk_rq_timeout(unsigned long timeout);
void blk_add_timer(struct request *req);
-bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
- unsigned int nr_segs, struct request **same_queue_rq);
bool blk_bio_list_merge(struct request_queue *q, struct list_head *list,
struct bio *bio, unsigned int nr_segs);
--
Jens Axboe
