[PATCH 5.19 158/717] blk-throttle: fix that io throttle can only work for single bio

Greg Kroah-Hartman
2022-10-22T08:08:19Z

From: Yu Kuai <yukuai3@huawei.com>

commit 320fb0f91e55ba248d4bad106b408e59099cfa89 upstream.

Test scripts:
cd /sys/fs/cgroup/blkio/
echo "8:0 1024" > blkio.throttle.write_bps_device
echo $$ > cgroup.procs
dd if=/dev/zero of=/dev/sda bs=10k count=1 oflag=direct &
dd if=/dev/zero of=/dev/sda bs=10k count=1 oflag=direct &

Test result:
10240 bytes (10 kB, 10 KiB) copied, 10.0134 s, 1.0 kB/s
10240 bytes (10 kB, 10 KiB) copied, 10.0135 s, 1.0 kB/s

The problem is that the second bio is finished after 10s instead of 20s.

Root cause:
1) second bio will be flagged:

__blk_throtl_bio
while (true) {
...
if (sq->nr_queued[rw]) -> some bio is throttled already
break
};
bio_set_flag(bio, BIO_THROTTLED); -> flag the bio

2) flagged bio will be dispatched without waiting:

throtl_dispatch_tg
tg_may_dispatch
tg_with_in_bps_limit
if (bps_limit == U64_MAX || bio_flagged(bio, BIO_THROTTLED))
*wait = 0; -> wait time is zero
return true;

commit 9f5ede3c01f9 ("block: throttle split bio in case of iops limit")
support to count split bios for iops limit, thus it adds flagged bio
checking in tg_with_in_bps_limit() so that split bios will only count
once for bps limit, however, it introduce a new problem that io throttle
won't work if multiple bios are throttled.

In order to fix the problem, handle iops/bps limit in different ways:

1) for iops limit, there is no flag to record if the bio is throttled,
and iops is always applied.
2) for bps limit, original bio will be flagged with BIO_BPS_THROTTLED,
and io throttle will ignore bio with the flag.

Noted this patch also remove the code to set flag in __bio_clone(), it's
introduced in commit 111be8839817 ("block-throttle: avoid double
charge"), and author thinks split bio can be resubmited and throttled
again, which is wrong because split bio will continue to dispatch from
caller.

Fixes: 9f5ede3c01f9 ("block: throttle split bio in case of iops limit")
Cc: <stable@vger.kernel.org>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220829022240.3348319-2-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
block/bio.c | 2 --
block/blk-throttle.c | 20 ++++++--------------
block/blk-throttle.h | 2 +-
include/linux/bio.h | 2 +-
include/linux/blk_types.h | 2 +-
5 files changed, 9 insertions(+), 19 deletions(-)

--- a/block/bio.c
+++ b/block/bio.c
@@ -760,8 +760,6 @@ EXPORT_SYMBOL(bio_put);
static int __bio_clone(struct bio *bio, struct bio *bio_src, gfp_t gfp)
{
bio_set_flag(bio, BIO_CLONED);
- if (bio_flagged(bio_src, BIO_THROTTLED))
- bio_set_flag(bio, BIO_THROTTLED);
bio->bi_ioprio = bio_src->bi_ioprio;
bio->bi_iter = bio_src->bi_iter;

--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -811,7 +811,7 @@ static bool tg_with_in_bps_limit(struct
unsigned int bio_size = throtl_bio_data_size(bio);

/* no need to throttle if this bio's bytes have been accounted */
- if (bps_limit == U64_MAX || bio_flagged(bio, BIO_THROTTLED)) {
+ if (bps_limit == U64_MAX || bio_flagged(bio, BIO_BPS_THROTTLED)) {
if (wait)
*wait = 0;
return true;
@@ -921,22 +921,13 @@ static void throtl_charge_bio(struct thr
unsigned int bio_size = throtl_bio_data_size(bio);

/* Charge the bio to the group */
- if (!bio_flagged(bio, BIO_THROTTLED)) {
+ if (!bio_flagged(bio, BIO_BPS_THROTTLED)) {
tg->bytes_disp[rw] += bio_size;
tg->last_bytes_disp[rw] += bio_size;
}

tg->io_disp[rw]++;
tg->last_io_disp[rw]++;
-
- /*
- * BIO_THROTTLED is used to prevent the same bio to be throttled
- * more than once as a throttled bio will go through blk-throtl the
- * second time when it eventually gets issued. Set it when a bio
- * is being charged to a tg.
- */
- if (!bio_flagged(bio, BIO_THROTTLED))
- bio_set_flag(bio, BIO_THROTTLED);
}

/**
@@ -1026,6 +1017,7 @@ static void tg_dispatch_one_bio(struct t
sq->nr_queued[rw]--;

throtl_charge_bio(tg, bio);
+ bio_set_flag(bio, BIO_BPS_THROTTLED);

/*
* If our parent is another tg, we just need to transfer @bio to
@@ -2159,8 +2151,10 @@ again:
qn = &tg->qnode_on_parent[rw];
sq = sq->parent_sq;
tg = sq_to_tg(sq);
- if (!tg)
+ if (!tg) {
+ bio_set_flag(bio, BIO_BPS_THROTTLED);
goto out_unlock;
+ }
}

/* out-of-limit, queue to @tg */
@@ -2189,8 +2183,6 @@ again:
}

out_unlock:
- bio_set_flag(bio, BIO_THROTTLED);
-
#ifdef CONFIG_BLK_DEV_THROTTLING_LOW
if (throttled || !td->track_bio_latency)
bio->bi_issue.value |= BIO_ISSUE_THROTL_SKIP_LATENCY;
--- a/block/blk-throttle.h
+++ b/block/blk-throttle.h
@@ -175,7 +175,7 @@ static inline bool blk_throtl_bio(struct
struct throtl_grp *tg = blkg_to_tg(bio->bi_blkg);

/* no need to throttle bps any more if the bio has been throttled */
- if (bio_flagged(bio, BIO_THROTTLED) &&
+ if (bio_flagged(bio, BIO_BPS_THROTTLED) &&
!(tg->flags & THROTL_TG_HAS_IOPS_LIMIT))
return false;

--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -509,7 +509,7 @@ static inline void bio_set_dev(struct bi
{
bio_clear_flag(bio, BIO_REMAPPED);
if (bio->bi_bdev != bdev)
- bio_clear_flag(bio, BIO_THROTTLED);
+ bio_clear_flag(bio, BIO_BPS_THROTTLED);
bio->bi_bdev = bdev;
bio_associate_blkg(bio);
}
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -323,7 +323,7 @@ enum {
BIO_QUIET, /* Make BIO Quiet */
BIO_CHAIN, /* chained bio, ->bi_remaining in effect */
BIO_REFFED, /* bio has elevated ->bi_cnt */
- BIO_THROTTLED, /* This bio has already been subjected to
+ BIO_BPS_THROTTLED, /* This bio has already been subjected to
* throttling rules. Don't do it again. */
BIO_TRACE_COMPLETION, /* bio_endio() should trace the final completion
* of this bio. */