[BUG] cgroupv2/blk: inconsistent I/O behavior in Cgroup v2 with set device wbps and wiops

[Lance Yang]

Hi all,

I've run into a problem with Cgroup v2 where it doesn't seem to correctly limit
I/O operations when I set both wbps and wiops for a device. However, if I only
set wbps, then everything works as expected.

To reproduce the problem, we can follow these command-based steps:

1. **System Information:**
   - Kernel Version and OS Release:
     ```
     $ uname -r
     6.10.0-rc5+

     $ cat /etc/os-release
     PRETTY_NAME="Ubuntu 24.04 LTS"
     NAME="Ubuntu"
     VERSION_ID="24.04"
     VERSION="24.04 LTS (Noble Numbat)"
     VERSION_CODENAME=noble
     ID=ubuntu
     ID_LIKE=debian
     HOME_URL="https://www.ubuntu.com/";
     SUPPORT_URL="https://help.ubuntu.com/";
     BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/";
     PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy";
     UBUNTU_CODENAME=noble
     LOGO=ubuntu-logo
     ```

2. **Device Information and Settings:**
   - List Block Devices and Scheduler:
     ```
     $ lsblk
     NAME    MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
     sda     8:0    0   4.4T  0 disk
     └─sda1  8:1    0   4.4T  0 part /data
     ...

     $ cat /sys/block/sda/queue/scheduler
     none [mq-deadline] kyber bfq

     $ cat /sys/block/sda/queue/rotational
     1
     ```

3. **Reproducing the problem:**
   - Navigate to the cgroup v2 filesystem and configure I/O settings:
     ```
     $ cd /sys/fs/cgroup/
     $ stat -fc %T /sys/fs/cgroup
     cgroup2fs
     $ mkdir test
     $ echo "8:0 wbps=10485760 wiops=100000" > io.max
     ```
     In this setup:
     wbps=10485760 sets the write bytes per second limit to 10 MB/s.
     wiops=100000 sets the write I/O operations per second limit to 100,000.

   - Add process to the cgroup and verify:
     ```
     $ echo $$ > cgroup.procs
     $ cat cgroup.procs
     3826771
     3828513
     $ ps -ef|grep 3826771
     root     3826771 3826768  0 22:04 pts/1    00:00:00 -bash
     root     3828761 3826771  0 22:06 pts/1    00:00:00 ps -ef
     root     3828762 3826771  0 22:06 pts/1    00:00:00 grep --color=auto 3826771
     ```

   - Observe I/O performance using `dd` commands and `iostat`:
     ```
     $ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
     $ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
     ```
     ```
     $ iostat -d 1 -h -y -p sda
     
	   tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
     7.00         0.0k         1.3M         0.0k       0.0k       1.3M       0.0k sda
     7.00         0.0k         1.3M         0.0k       0.0k       1.3M       0.0k sda1


      tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
     5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda
     5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda1


      tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
    21.00         0.0k         1.4M         0.0k       0.0k       1.4M       0.0k sda
    21.00         0.0k         1.4M         0.0k       0.0k       1.4M       0.0k sda1


      tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
     5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda
     5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda1


      tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
     5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda
     5.00         0.0k         1.2M         0.0k       0.0k       1.2M       0.0k sda1


      tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
  1848.00         0.0k       448.1M         0.0k       0.0k     448.1M       0.0k sda
  1848.00         0.0k       448.1M         0.0k       0.0k     448.1M       0.0k sda1
     ```
Initially, the write speed is slow (<2MB/s) then suddenly bursts to several
hundreds of MB/s.

   - Testing with wiops set to max:
     ```
     echo "8:0 wbps=10485760 wiops=max" > io.max
     $ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
     $ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
     ```
     ```
     $ iostat -d 1 -h -y -p sda

      tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
    48.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda
    48.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda1


      tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
    40.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda
    40.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda1


      tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
    41.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda
    41.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda1


      tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
    46.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda
    46.00         0.0k        10.0M         0.0k       0.0k      10.0M       0.0k sda1


      tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd Device
    55.00         0.0k        10.2M         0.0k       0.0k      10.2M       0.0k sda
    55.00         0.0k        10.2M         0.0k       0.0k      10.2M       0.0k sda1
     ```
The iostat output shows the write operations as stabilizing at around 10 MB/s,
which aligns with the defined limit of 10 MB/s. After setting wiops to max, the
I/O limits appear to work as expected. 


Thanks,
Lance