Re: [PATCH v7 3/5] cpuset: Add a root-only cpus.isolated v2 control file

Peter Zijlstra
2018-05-02T14:08:52Z

On Thu, Apr 19, 2018 at 09:47:02AM -0400, Waiman Long wrote:
> diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
> index c970bd7..8d89dc2 100644
> --- a/Documentation/cgroup-v2.txt
> +++ b/Documentation/cgroup-v2.txt
> @@ -1484,6 +1484,31 @@ Cpuset Interface Files
> a subset of "cpuset.cpus". Its value will be affected by CPU
> hotplug events.
>
> + cpuset.cpus.isolated
> + A read-write multiple values file which exists on root cgroup
> + only.
> +
> + It lists the CPUs that have been withdrawn from the root cgroup
> + for load balancing. These CPUs can still be allocated to child
> + cpusets with load balancing enabled, if necessary.
> +
> + If a child cpuset contains only an exclusive set of CPUs that are
> + a subset of the isolated CPUs and with load balancing enabled,
> + these CPUs will be load balanced on a separate root domain from
> + the one in the root cgroup.
> +
> + Just putting the CPUs into "cpuset.cpus.isolated" will be
> + enough to disable load balancing on those CPUs as long as they
> + do not appear in a child cpuset with load balancing enabled.
> + Fine-grained control of cpu isolation can also be done by
> + putting these isolated CPUs into child cpusets with load
> + balancing disabled.
> +
> + The "cpuset.cpus.isolated" should be set up before child
> + cpusets are created. Once child cpusets are present, changes
> + to "cpuset.cpus.isolated" will not be allowed if the CPUs that
> + change their states are in any of the child cpusets.
> +

So I see why you did this, but it is _really_ ugly and breaks the
container invariant.

Ideally we'd make the root group less special, not more special.