Re: [RFC PATCH v2 13/17] cgroup: Allow fine-grained controllers control in cgroup v2

[Waiman Long <longman@redhat.com>]

On 05/24/2017 01:31 PM, Tejun Heo wrote:
> Hello, Waiman.
>
> On Fri, May 19, 2017 at 05:20:01PM -0400, Waiman Long wrote:
>>> This breaks the invariant that in a cgroup its resource control knobs
>>> control distribution of resources from its parent.  IOW, the resource
>>> control knobs of a cgroup always belong to the parent.  This is also
>>> reflected in how delegation is done.  The delegatee assumes ownership
>>> of the cgroup itself and the ability to manage sub-cgroups but doesn't
>>> get the ownership of the resource control knobs as otherwise the
>>> parent would lose control over how it distributes its resources.
>> One twist that I am thinking is to have a controller enabled by the
>> parent in subtree_control, but then allow the child to either disable it
>> or set it in pass-through mode by writing to controllers file. IOW, a
>> child cannot enable a controller without parent's permission. Once a
>> child has permission, it can do whatever it wants. A parent cannot force
>> a child to have a controller enabled.
> Heh, I think I need more details to follow your proposal.  Anyways,
> what we need to guarantee is that a descendant is never allowed to
> pull in more resources than its ancestors want it to.

What I am saying is as follows:
    / A
P - B
   \ C

# echo +memory > P/cgroups.subtree_control
# echo -memory > P/A/cgroup.controllers
# echo "#memory" > P/B/cgroup.controllers

The parent grants the memory controller to its children - A, B and C.
Child A has the memory controller explicitly disabled. Child B has the
memory controller in pass-through mode, while child C has the memory
controller enabled by default. "echo +memory > cgroup.controllers" is
not allowed. There are 2 possible choices with regard to the '-' or '#'
prefixes. We can allow them before the grant from the parent or only
after that. In the former case, the state remains dormant until after
the grant from the parent.

Cheers,
Longman