linux-kernel - [PATCH V3] bcache: dont reset bio opf in bch_data_insert

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Message-Id: <20210127031550.3605-1-dongsheng.yang@easystack.cn>
Date:   Wed, 27 Jan 2021 11:15:50 +0800
From:   Dongsheng Yang <dongsheng.yang@...ystack.cn>
To:     colyli@...e.de, linux-bcache@...r.kernel.org,
        linux-kernel@...r.kernel.org, mchristi@...hat.com
Cc:     Dongsheng Yang <dongsheng.yang@...ystack.cn>
Subject: [PATCH V3] bcache: dont reset bio opf in bch_data_insert_start

commit ad0d9e76(bcache: use bio op accessors) makes the bi_opf
modified by bio_set_op_attrs(). But there is a logical
problem in this commit:

                trace_bcache_cache_insert(k);
                bch_keylist_push(&op->insert_keys);

-               n->bi_rw |= REQ_WRITE;
+               bio_set_op_attrs(n, REQ_OP_WRITE, 0);
                bch_submit_bbio(n, op->c, k, 0);
        } while (n != bio);

The old code add REQ_WRITE into bio n and keep other flags; the
new code set REQ_OP_WRITE to bi_opf, but reset all other flags.

This problem is discoverd in our performance testing:
(1) start a fio with 1M x 128depth for read in /dev/nvme0n1p1
(2) start a fio with 1M x 128depth for write in /dev/escache0 (cache
device is /dev/nvme0n1p2)

We found the BW of reading is 2000+M/s, but the BW of writing is
0-100M/s. After some debugging, we found the problem is io submit in
writting is very slow.

bch_data_insert_start() insert a bio to /dev/nvme0n1p1, but as
cached_dev submit stack bio will be added into current->bio_list, and
return.Then __submit_bio_noacct() will submit the new bio in bio_list
into /dev/nvme0n1p1. This operation would be slow in
blk_mq_submit_bio() -> rq_qos_throttle(q, bio);

The rq_qos_throttle() will call wbt_should_throttle(),
static inline bool wbt_should_throttle(struct rq_wb *rwb, struct bio *bio)
{
        switch (bio_op(bio)) {
        case REQ_OP_WRITE:
                /*
                 * Don't throttle WRITE_ODIRECT
                 */
                if ((bio->bi_opf & (REQ_SYNC | REQ_IDLE)) ==
                    (REQ_SYNC | REQ_IDLE))
                        return false;
... ...
}

As the bio_set_op_attrs() reset the (REQ_SYNC | REQ_IDLE), so this write
bio will be considered as non-direct write.

After this fix, bio to nvme will flaged as (REQ_SYNC | REQ_IDLE),
then fio for writing will get about 1000M/s bandwidth.

Fixes: ad0d9e76a412 ("bcache: use bio op accessors")
CC: Mike Christie <mchristi@...hat.com>
Signed-off-by: Dongsheng Yang <dongsheng.yang@...ystack.cn>
---
V3-V2:
	remove an unused close-line in commit message.

 drivers/md/bcache/request.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index c7cadaafa947..eb734f7ddaac 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -244,7 +244,7 @@ static void bch_data_insert_start(struct closure *cl)
 		trace_bcache_cache_insert(k);
 		bch_keylist_push(&op->insert_keys);

-		bio_set_op_attrs(n, REQ_OP_WRITE, 0);
+		n->bi_opf |= REQ_OP_WRITE;
 		bch_submit_bbio(n, op->c, k, 0);
 	} while (n != bio);

-- 
2.25.1