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Subjectcgroup: Fix split bio been throttled more than once
Hi Tejun,

These days, we have tested the cgroup blkio throttle function use fio,
and we found a problem that when we use buffered IO or set the big block
size like 1M, then the IO performance cannot reach the value we set.
For example we set blkio.throttle.read_bps_device as 10M, in kernel
version 4.3 IO performance can only reach 6M, and in kernel version
4.4 the actual IO bps is only 3.1M.

Then we did some research and find that in kernel version 4.3 brought in
blk_queue_split() function to split the big size bio into several parts,
and some of them are calling the generic_make_request() again, this result
the bio been throttled more than once. so the actual bio sent to device is
less than we expected.

We have checked the newest kernel of 4.7-rc5, this problem is still exist.

Based on this kind of situation, we propose a fix solution to add a flag bit
in bio to let the splited bio bypass the blk_queue_split(). Below is the patch
we used to fix this problem.

Thanks

From b5ea98c9fc5612f9390b65bd9cf4ff344b6cfe92 Mon Sep 17 00:00:00 2001
From: Jiale Li <aaronlee0817@163.com>
Date: Mon, 4 Jul 2016 09:23:32 -0400
Subject: [PATCH] cgroup: Fix split bio been throttled more than once

From kernel version 4.3, blk_queue_split() been added into the kernel,
it calls the generic_make_request() function, witch means a part of
the bio will be counted more than once in blk_throtl_bio(). This result
the throttle function of cgroup cannot work as we expected.

This patch add a new flag bit in bio, to let the split bio bypass the
blk_throtl_bio().

Signed-off-by: Jiale Li <aaronlee0817@163.com>
---
block/blk-merge.c | 1 +
block/blk-throttle.c | 4 ++++
include/linux/blk_types.h | 1 +
3 files changed, 6 insertions(+)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 2613531..7b17a65 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -190,6 +190,7 @@ void blk_queue_split(struct request_queue *q, struct bio **bio,

bio_chain(split, *bio);
trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
+ bio_set_flag(*bio, BIO_SPLITED);
generic_make_request(*bio);
*bio = split;
}
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 47a3e54..4ffde95 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1403,6 +1403,10 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,

WARN_ON_ONCE(!rcu_read_lock_held());

+ /* if the bio has been splited, should not throttle again */
+ if (bio_flagged(bio, BIO_SPLITED))
+ goto out;
+
/* see throtl_charge_bio() */
if ((bio->bi_rw & REQ_THROTTLED) || !tg->has_rules[rw])
goto out;
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 77e5d81..b294780 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -120,6 +120,7 @@ struct bio {
#define BIO_QUIET 6 /* Make BIO Quiet */
#define BIO_CHAIN 7 /* chained bio, ->bi_remaining in effect */
#define BIO_REFFED 8 /* bio has elevated ->bi_cnt */
+#define BIO_SPLITED 9 /* bio has been splited */

/*
* Flags starting here get preserved by bio_reset() - this includes
--
1.9.1
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