Commit 05f1dd53 authored by Jens Axboe's avatar Jens Axboe
Browse files

block: add queue flag for disabling SG merging 



If devices are not SG starved, we waste a lot of time potentially
collapsing SG segments. Enough that 1.5% of the CPU time goes
to this, at only 400K IOPS. Add a queue flag, QUEUE_FLAG_NO_SG_MERGE,
which just returns the number of vectors in a bio instead of looping
over all segments and checking for collapsible ones.

Add a BLK_MQ_F_SG_MERGE flag so that drivers can opt-in on the sg
merging, if they so desire.

Signed-off-by: default avatarJens Axboe <axboe@fb.com>
parent 4d92a9be

block/blk-merge.c

+21 −7
Original line number Diff line number Diff line
@@ -13,7 +13,7 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q, 
					     struct bio *bio)

{

	struct bio_vec bv, bvprv = { NULL };

	int cluster, high, highprv = 1;

	int cluster, high, highprv = 1, no_sg_merge;

	unsigned int seg_size, nr_phys_segs;

	struct bio *fbio, *bbio;

	struct bvec_iter iter;
@@ -35,12 +35,21 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q, 
	cluster = blk_queue_cluster(q);

	seg_size = 0;

	nr_phys_segs = 0;

	no_sg_merge = test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags);

	high = 0;

	for_each_bio(bio) {

		bio_for_each_segment(bv, bio, iter) {

			/*

			 * If SG merging is disabled, each bio vector is

			 * a segment

			 */

			if (no_sg_merge)

				goto new_segment;



			/*

			 * the trick here is making sure that a high page is

			 * never considered part of another segment, since that

			 * might change with the bounce page.

			 * never considered part of another segment, since

			 * that might change with the bounce page.

			 */

			high = page_to_pfn(bv.bv_page) > queue_bounce_pfn(q);

			if (!high && !highprv && cluster) {
@@ -84,11 +93,16 @@ void blk_recalc_rq_segments(struct request *rq) 


void blk_recount_segments(struct request_queue *q, struct bio *bio)

{

	if (test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags))

		bio->bi_phys_segments = bio->bi_vcnt;

	else {

		struct bio *nxt = bio->bi_next;



		bio->bi_next = NULL;

		bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio);

		bio->bi_next = nxt;

	}



	bio->bi_flags |= (1 << BIO_SEG_VALID);

}

EXPORT_SYMBOL(blk_recount_segments);

block/blk-mq.c

+3 −0
Original line number Diff line number Diff line
@@ -1829,6 +1829,9 @@ struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set) 
	q->mq_ops = set->ops;

	q->queue_flags |= QUEUE_FLAG_MQ_DEFAULT;



	if (!(set->flags & BLK_MQ_F_SG_MERGE))

		q->queue_flags |= 1 << QUEUE_FLAG_NO_SG_MERGE;



	q->sg_reserved_size = INT_MAX;



	INIT_WORK(&q->requeue_work, blk_mq_requeue_work);

include/linux/blk-mq.h

+1 −0
Original line number Diff line number Diff line
@@ -129,6 +129,7 @@ enum { 
	BLK_MQ_F_SHOULD_MERGE	= 1 << 0,

	BLK_MQ_F_SHOULD_SORT	= 1 << 1,

	BLK_MQ_F_TAG_SHARED	= 1 << 2,

	BLK_MQ_F_SG_MERGE	= 1 << 3,



	BLK_MQ_S_STOPPED	= 0,

	BLK_MQ_S_TAG_ACTIVE	= 1,

include/linux/blkdev.h

+1 −0
Original line number Diff line number Diff line
@@ -510,6 +510,7 @@ struct request_queue { 
#define QUEUE_FLAG_SAME_FORCE  18	/* force complete on same CPU */

#define QUEUE_FLAG_DEAD        19	/* queue tear-down finished */

#define QUEUE_FLAG_INIT_DONE   20	/* queue is initialized */

#define QUEUE_FLAG_NO_SG_MERGE 21	/* don't attempt to merge SG segments*/



#define QUEUE_FLAG_DEFAULT	((1 << QUEUE_FLAG_IO_STAT) |		\

				 (1 << QUEUE_FLAG_STACKABLE)	|	\

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