Commit 5e84ea3a authored by Jens Axboe's avatar Jens Axboe
Browse files

block: attempt to merge with existing requests on plug flush 



One of the disadvantages of on-stack plugging is that we potentially
lose out on merging since all pending IO isn't always visible to
everybody. When we flush the on-stack plugs, right now we don't do
any checks to see if potential merge candidates could be utilized.

Correct this by adding a new insert variant, ELEVATOR_INSERT_SORT_MERGE.
It works just ELEVATOR_INSERT_SORT, but first checks whether we can
merge with an existing request before doing the insertion (if we fail
merging).

This fixes a regression with multiple processes issuing IO that
can be merged.

Thanks to Shaohua Li <shaohua.li@intel.com> for testing and fixing
an accounting bug.

Signed-off-by: default avatarJens Axboe <jaxboe@fusionio.com>
parent 4345caba

block/blk-core.c

+1 −1
Original line number Diff line number Diff line
@@ -2685,7 +2685,7 @@ static void flush_plug_list(struct blk_plug *plug) 
		/*

		 * rq is already accounted, so use raw insert

		 */

		__elv_add_request(q, rq, ELEVATOR_INSERT_SORT);

		__elv_add_request(q, rq, ELEVATOR_INSERT_SORT_MERGE);

	}



	if (q) {

block/blk-merge.c

+6 −0
Original line number Diff line number Diff line
@@ -465,3 +465,9 @@ int attempt_front_merge(struct request_queue *q, struct request *rq) 


	return 0;

}



int blk_attempt_req_merge(struct request_queue *q, struct request *rq,

			  struct request *next)

{

	return attempt_merge(q, rq, next);

}

block/blk.h

+2 −0
Original line number Diff line number Diff line
@@ -103,6 +103,8 @@ int ll_front_merge_fn(struct request_queue *q, struct request *req, 
		      struct bio *bio);

int attempt_back_merge(struct request_queue *q, struct request *rq);

int attempt_front_merge(struct request_queue *q, struct request *rq);

int blk_attempt_req_merge(struct request_queue *q, struct request *rq,

				struct request *next);

void blk_recalc_rq_segments(struct request *rq);

void blk_rq_set_mixed_merge(struct request *rq);

block/elevator.c

+49 −3
Original line number Diff line number Diff line
@@ -521,6 +521,40 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio) 
	return ELEVATOR_NO_MERGE;

}



/*

 * Attempt to do an insertion back merge. Only check for the case where

 * we can append 'rq' to an existing request, so we can throw 'rq' away

 * afterwards.

 *

 * Returns true if we merged, false otherwise

 */

static bool elv_attempt_insert_merge(struct request_queue *q,

				     struct request *rq)

{

	struct request *__rq;



	if (blk_queue_nomerges(q))

		return false;



	/*

	 * First try one-hit cache.

	 */

	if (q->last_merge && blk_attempt_req_merge(q, q->last_merge, rq))

		return true;



	if (blk_queue_noxmerges(q))

		return false;



	/*

	 * See if our hash lookup can find a potential backmerge.

	 */

	__rq = elv_rqhash_find(q, blk_rq_pos(rq));

	if (__rq && blk_attempt_req_merge(q, __rq, rq))

		return true;



	return false;

}



void elv_merged_request(struct request_queue *q, struct request *rq, int type)

{

	struct elevator_queue *e = q->elevator;
@@ -538,14 +572,18 @@ void elv_merge_requests(struct request_queue *q, struct request *rq, 
			     struct request *next)

{

	struct elevator_queue *e = q->elevator;

	const int next_sorted = next->cmd_flags & REQ_SORTED;



	if (e->ops->elevator_merge_req_fn)

	if (next_sorted && e->ops->elevator_merge_req_fn)

		e->ops->elevator_merge_req_fn(q, rq, next);



	elv_rqhash_reposition(q, rq);

	elv_rqhash_del(q, next);



	if (next_sorted) {

		elv_rqhash_del(q, next);

		q->nr_sorted--;

	}



	q->last_merge = rq;

}
@@ -647,6 +685,14 @@ void elv_insert(struct request_queue *q, struct request *rq, int where) 
		__blk_run_queue(q, false);

		break;



	case ELEVATOR_INSERT_SORT_MERGE:

		/*

		 * If we succeed in merging this request with one in the

		 * queue already, we are done - rq has now been freed,

		 * so no need to do anything further.

		 */

		if (elv_attempt_insert_merge(q, rq))

			break;

	case ELEVATOR_INSERT_SORT:

		BUG_ON(rq->cmd_type != REQ_TYPE_FS &&

		       !(rq->cmd_flags & REQ_DISCARD));

include/linux/elevator.h

+1 −0
Original line number Diff line number Diff line
@@ -166,6 +166,7 @@ extern struct request *elv_rb_find(struct rb_root *, sector_t); 
#define ELEVATOR_INSERT_SORT	3

#define ELEVATOR_INSERT_REQUEUE	4

#define ELEVATOR_INSERT_FLUSH	5

#define ELEVATOR_INSERT_SORT_MERGE	6



/*

 * return values from elevator_may_queue_fn

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