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

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Message-ID: <20170712074555.slefkebvdpfjse34@hirez.programming.kicks-ass.net>
Date:   Wed, 12 Jul 2017 09:45:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
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 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.