linux-kernel - Re: [RFC PATCH v2 11/17] cgroup: Implement new thread mode semantics

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Date:   Thu, 1 Jun 2017 14:44:48 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.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 11/17] cgroup: Implement new thread mode semantics

On 06/01/2017 11:10 AM, Peter Zijlstra wrote:
> On Thu, Jun 01, 2017 at 10:50:42AM -0400, Tejun Heo wrote:
>> Hello, Waiman.
>>
>> A short update.  I tried making root special while keeping the
>> existing threaded semantics but I didn't really like it because we
>> have to couple controller enables/disables with threaded
>> enables/disables.  I'm now trying a simpler, albeit a bit more
>> tedious, approach which should leave things mostly symmetrical.  I'm
>> hoping to be able to post mostly working patches this week.
> I've not had time to look at any of this. But the question I'm most
> curious about is how cgroup-v2 preserves the container invariant.
>
> That is, each container (namespace) should look like a 'real' machine.
> So just like userns allows to have a uid-0 (aka root) for each container
> and pidns allows a pid-1 for each container, cgroupns should provide a
> root group for each container.
>
> And cgroup-v2 has this 'exception' (aka wart) for the root group which
> needs to be replicated for each namespace.

One of the changes that I proposed in my patches was to get rid of the
no internal process constraint. I think that will solve a big part of
the container invariant problem that we have with cgroup v2.

Cheers,
Longman