Cannot move certain kernel threads from root cgroup
 

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.

You can't do squat with a kernel thread!

A kernel thread is part of the kernel. (Duh.) It is "a Godly Immortal, not of this Mortal Realm, with doth but deign to appear in the presence of Men." This mechanism exists merely for the convenience of The Masters of This Universe, who have found it convenient to take upon themselves "a robe of flesh" that they may, for instance, execute I/O operations and wait for those to be completed, without inventing an entirely new metaphor by which to do so.

Ye may glimpse these immortal beings as they flash to-and-fro among ye, but ye may not touch them.