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

[Lance Yang <ioworker0@gmail.com>]

Hi Michal,

Thanks a lot for jumping in!

On Mon, Aug 12, 2024 at 11:43 PM Michal Koutný <mkoutny@suse.com> wrote:
>
> +Cc Kuai
>
> On Mon, Aug 12, 2024 at 11:00:30PM GMT, Lance Yang <ioworker0@gmail.com> wrote:
> > 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.
>
> What it would be on average?
> IOW how long would the whole operation in throttled cgroup take?
>
> >
> >    - 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
>
> Thanks for the report Lance. Is this something you started seeing after
> a kernel update or switch to cgroup v2? (Or you simply noticed with this
> setup only?)

I just switched to cgroup v2 to begin testing, as we intend to have
containers run
in cgroup v2. Testing on both the 5.14.0 and mainline versions ;)

Thanks again for your time!
Lance

>
>
> Michal