From 05f1dd5315217398fc8d122bdee80f96a9f21274 Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@fb.com>
Date: Thu, 29 May 2014 09:53:32 -0600
Subject: [PATCH] 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: Jens Axboe <axboe@fb.com>
---
block/blk-merge.c | 28 +++++++++++++++++++++-------
block/blk-mq.c | 3 +++
include/linux/blk-mq.h | 1 +
include/linux/blkdev.h | 1 +
4 files changed, 26 insertions(+), 7 deletions(-)
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 6c583f9c5b65..b3bf0df0f4c2 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -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)
{
- struct bio *nxt = bio->bi_next;
+ 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_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);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index f27fe44230c2..f98d977fd150 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -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);
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 91dfb75ce39f..95de239444d2 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -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,
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 098304576d51..695b9fd41efe 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -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) | \