Change cgroups of subprocess in docker

I ran a docker container with several process in it. I want to limit the iops/bps of a child process and I try to use cgroups to do that, but failed.

I create a cgroups by

mkdir /sys/fs/cgroup/blkio/example
echo "7:3 1048576" > /sys/fs/cgroup/blkio/example/blkio.throttle.read_bps_device
# use `ps aux` to find out the pid
echo <pid> > /sys/fs/cgroup/blkio/example/tasks

When I use iotop to get the io status, I found the DISK READ of the process not limited.
However, if a process running on host is added to that cgroups, it works.

I want to know is there something wrong in my steps using cgroups, and is it a reasonable approach to use cgroups in docker.

Source: StackOverflow