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 9f5ede3c ("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 111be883 ("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: 9f5ede3c ("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.comSigned-off-by: Jens Axboe <axboe@kernel.dk>