Cgroups: Cannot move certain kernel threads from root cgroup

[Avinash Chiganmi <avinash.ca@xxxxxxxxx>]

Hi,

I created a "cpu" cgroup:
     mkdir /dev/cgroup
     mkdir /dev/cgroup/cpu
     mount -t cgroup -o cpu cpu /dev/cgroup/cpu
     cd /dev/cgroup/cpu/
     mkdir low

Now, I try to move tasks to the sub-cgroup "low":
     for task in `cat /dev/cgroup/cpu/tasks`; do   /bin/echo $task >
/dev/cgroup/cpu/low/tasks || echo "Failed to move PID $task"; done

/bin/echo: write error: Invalid argument
Failed to move PID 3
/bin/echo: write error: Invalid argument
Failed to move PID 4
/bin/echo: write error: Invalid argument
Failed to move PID 5
/bin/echo: write error: Invalid argument
Failed to move PID 6
/bin/echo: write error: Invalid argument
Failed to move PID 7
/bin/echo: write error: Invalid argument
Failed to move PID 8
/bin/echo: write error: Invalid argument
.
.
.
/bin/echo: write error: Invalid argument
Failed to move PID 58

I was able to move the "init" process itself. However, I failed to move
specific kernel threads:

root         3     2  0 18:16 ?        00:00:00 [migration/0]
root         4     2  0 18:16 ?        00:00:00 [sirq-high/0]
root         5     2  0 18:16 ?        00:00:04 [sirq-timer/0]
root         6     2  0 18:16 ?        00:00:00 [sirq-net-tx/0]
root         7     2  0 18:16 ?        00:00:00 [sirq-net-rx/0]
root         8     2  0 18:16 ?        00:00:00 [sirq-block/0]
root         9     2  0 18:16 ?        00:00:00 [sirq-block-iopo]
root        10     2  0 18:16 ?        00:00:00 [sirq-tasklet/0]
root        11     2  0 18:16 ?        00:00:00 [sirq-sched/0]
root        12     2  0 18:16 ?        00:00:00 [sirq-hrtimer/0]
root        13     2  0 18:16 ?        00:00:04 [sirq-rcu/0]
root        14     2  0 18:16 ?        00:00:00 [watchdog/0]
root        15     2  0 18:16 ?        00:00:00 [desched/0]
root        16     2  0 18:16 ?        00:00:00 [migration/1]
root        17     2  0 18:16 ?        00:00:00 [sirq-high/1]
root        18     2  0 18:16 ?        00:00:03 [sirq-timer/1]
root        19     2  0 18:16 ?        00:00:00 [sirq-net-tx/1]
root        20     2  0 18:16 ?        00:00:00 [sirq-net-rx/1]
root        21     2  0 18:16 ?        00:00:00 [sirq-block/1]


Is there something special about these threads that prevents them from
being moved to a different cgroup? Is there something I am missing?

Thanks!
Avi.
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