linux-kernel - [PATCH] block: blk-merge: fast-clone bio when splitting rw bios

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Message-Id: <1442502807-24377-1-git-send-email-ming.lei@canonical.com>
Date:	Thu, 17 Sep 2015 23:13:27 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Cc:	Ming Lei <ming.lei@...onical.com>,
	Christoph Hellwig <hch@...radead.org>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Ming Lin <ming.l@....samsung.com>,
	Dongsu Park <dpark@...teo.net>
Subject: [PATCH] block: blk-merge: fast-clone bio when splitting rw bios

biovecs has become immutable since v3.13, so it isn't necessary
to allocate biovecs for the new cloned bios, then we can save
one extra biovecs allocation/copy, and the allocation is often
not fixed-length and a bit more expensive.

For example, if the 'max_sectors_kb' of null blk's queue is set
as 16(32 sectors) via sysfs just for making more splits, this patch
can increase throught about ~70% in the sequential read test over
null_blk(direct io, bs: 1M).

Cc: Christoph Hellwig <hch@...radead.org>
Cc: Kent Overstreet <kent.overstreet@...il.com>
Cc: Ming Lin <ming.l@....samsung.com>
Cc: Dongsu Park <dpark@...teo.net>
Signed-off-by: Ming Lei <ming.lei@...onical.com>
---
 block/blk-merge.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index d088cff..be2493f 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -66,16 +66,13 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 					 struct bio *bio,
 					 struct bio_set *bs)
 {
-	struct bio *split;
 	struct bio_vec bv, bvprv;
 	struct bvec_iter iter;
 	unsigned seg_size = 0, nsegs = 0, sectors = 0;
 	int prev = 0;
 
 	bio_for_each_segment(bv, bio, iter) {
-		sectors += bv.bv_len >> 9;
-
-		if (sectors > queue_max_sectors(q))
+		if (sectors + (bv.bv_len >> 9) > queue_max_sectors(q))
 			goto split;
 
 		/*
@@ -96,6 +93,7 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 			seg_size += bv.bv_len;
 			bvprv = bv;
 			prev = 1;
+			sectors += bv.bv_len >> 9;
 			continue;
 		}
 new_segment:
@@ -106,21 +104,12 @@ new_segment:
 		bvprv = bv;
 		prev = 1;
 		seg_size = bv.bv_len;
+		sectors += bv.bv_len >> 9;
 	}
 
 	return NULL;
 split:
-	split = bio_clone_bioset(bio, GFP_NOIO, bs);
-
-	split->bi_iter.bi_size -= iter.bi_size;
-	bio->bi_iter = iter;
-
-	if (bio_integrity(bio)) {
-		bio_integrity_advance(bio, split->bi_iter.bi_size);
-		bio_integrity_trim(split, 0, bio_sectors(split));
-	}
-
-	return split;
+	return bio_split(bio, sectors, GFP_NOIO, bs);
 }
 
 void blk_queue_split(struct request_queue *q, struct bio **bio,
-- 
1.9.1

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