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SubjectRe: cgroup scheduling: Adding kthreadd to a non-RT cgroup can deadlock the kernel
Ok. I bothered reporting this here since the kernel /does/ try to prevent you
from shooting yourself in the foot in related ways, by moving existing
SCHED_FIFO tasks into a cgroup without any RT runtime, so I figured it might
make sense to add a check here. libcgroup, for example, will try to move all
processes into a default cgroup, ignoring errors, and relies on the kernel to
prevent it from shooting itself in the foot.

It sounds like you consider that behavior a bug, though, so I'll go report this
bug there.

- Nelson

On Wed, Jan 5, 2011 at 4:44 AM, Peter Zijlstra <> wrote:
> On Tue, 2011-01-04 at 23:54 -0500, Nelson Elhage wrote:
>> Hi,
>> I've found a bug where, on CONFIG_RT_GROUP_SCHED systems, adding the kthreadd
>> task to a cgroup with cpu.rt_runtime_us = 0 (as some cgroup configuration
>> scripts do, when they move all processes into a default cgroup), can result in
>> deadlocks in the kernel.
> IMHO its a bug to move kthreadd into a cgroup. Simply don't do that.

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