linux-kernel - Re: [RFC PATCH-cgroup 4/6] cgroup: Introduce subtree root mode

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Message-ID: <20170621213807.GC14720@htj.duckdns.org>
Date:   Wed, 21 Jun 2017 17:38:07 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
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, kernel-team@...com, pjt@...gle.com,
        luto@...capital.net, efault@....de, torvalds@...ux-foundation.org
Subject: Re: [RFC PATCH-cgroup 4/6] cgroup: Introduce subtree root mode

Hello, Waiman.

On Wed, Jun 14, 2017 at 11:05:35AM -0400, Waiman Long wrote:
> Subtree root mode is a new cgroup mode which applies the following
> restrictions when turned on:
> 
>  1) Controllers are only allowed to be passed to the children in
>     bypass mode except those with the "enable_on_root" flag on.
>  2) Only 1 child cgroup is allowed.
> 
> That lone child can be used as the pseudo root of a container cgroup
> hierarchy.  All the resources, if controlled, are in the parent
> cgroup. There will be no control knobs in the child. That makes it
> look and feel like a root.

I'm not sure not having the control files in the child makes that much
difference.

> That pseudo root is also considered to be mixable and so can become
> root of a mixed threaded subtree. The no internal process constraint
> also does not apply.

Heh, you can't just declare a non-root cgroup to be a mixed root but
if you're special casing this and making the kernel play a masquerade
with special node, you can make cgroup provide a mixed root while
hosting the internal processes in a dedicated leaf cgroup which isn't
visible to the nested root, right?

It's all a game of masquerading tho and doesn't actually enable
anything which isn't possible now.  This would definitely be useful
for testing.

Thanks.

-- 
tejun