linux-kernel - Re: [PATCHSET for-4.13] cgroup: implement cgroup2 thread mode, v2

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Message-ID: <0f531185-4d39-ff50-8a48-067d6d209673@redhat.com>
Date:   Wed, 12 Jul 2017 10:00:34 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        hannes@...xchg.org, mingo@...hat.com, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com, pjt@...gle.com,
        luto@...capital.net, efault@....de, torvalds@...ux-foundation.org
Subject: Re: [PATCHSET for-4.13] cgroup: implement cgroup2 thread mode, v2

On 07/12/2017 03:45 AM, Peter Zijlstra wrote:
> On Tue, Jul 11, 2017 at 05:12:39PM -0400, Waiman Long wrote:
>> On 07/11/2017 12:52 PM, Peter Zijlstra wrote:
>>> On Tue, Jul 11, 2017 at 10:14:42AM -0400, Waiman Long wrote:
>>>
>>>> The "join" was a special op for the children of cgroup root to join the
>>>> root as part of a threaded subtree. The children can instead use the
>>>> "enable" option to become a thread root which was the configuration
>>>> shown above.  This behavior applied only to children of root. Down the
>>>> hierarchy, you can't have configuration like:
>>>>
>>>>      R (t=0)
>>>>     / \
>>>>        D (t=1)
>>>>       / \
>>>>      T   D (t=1)
>>> Why not?
>>>
>>> First you create:
>>>
>>>       R (t=0)
>>>      / \
>>>         D (t=1)
>>>        / \
>>>       T   T (t=1)
>>>
>>> Then you flip t=0 like:
>>>
>>>       R (t=0)
>>>      / \
>>>         D (t=1)
>>>        / \
>>>       T   D (t=0)
>>>
>>> And then you flip t=1 again:
>>>
>>>       R (t=0)
>>>      / \
>>>         D (t=1)
>>>        / \
>>>       T   D (t=1)
>> Tejun's thread mode patch has constraints on what operations are allowed
>> and what aren't. For a threaded subtree, thread mode cannot be disabled
>> in the middle of the tree.
> Where in that scenario did I change anything in the middle? All
> operations were on a leaf group.

What I mean is that you can't disable thread mode if not at the thread
root with no children left.

Cheers,
Longman