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

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Date:   Wed, 24 May 2017 14:17:50 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, kernel-team@...com, pjt@...gle.com,
        luto@...capital.net, efault@....de
Subject: Re: [RFC PATCH v2 13/17] cgroup: Allow fine-grained controllers
 control in cgroup v2

On 05/24/2017 01:56 PM, Tejun Heo wrote:
> Hello,
>
> On Wed, May 24, 2017 at 01:49:46PM -0400, Waiman Long wrote:
>> 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.
> Ah, I see, you want cgroup.controllers to be able to mask available
> controllers by the parent.  Can you expand your example with further
> nesting and how #memory on cgroup.controllers would affect the nested
> descendant?
>
> Thanks.
>
I would allow enabling the controller in subtree_control if granted from
the parent and not explicitly disabled. IOW, both B and C can "echo
+memory" to their subtree_control to grant memory controller to their
children, but not A. A has to re-enable memory controller or set it to
pass-through mode before it can enable it in subtree_control. I need to
clarify that "echo +memory > cgroup.controllers" is allowed to re-enable
it, but not without the granting from its parent.

Cheers,
Longman