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Date:   Wed, 2 Dec 2020 09:02:11 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Don <joshdon@google.com>
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
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Subject: Re: [PATCH -tip 26/32] sched: Add a second-level tag for nested
 CGroup usecase
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On Tue, Dec 01, 2020 at 10:18:00PM -0800, Josh Don wrote:
> Hey Peter,
> 
> On Wed, Nov 25, 2020 at 5:43 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Why can't the above work by setting 'tag' (that's a terrible name, why
> > does that still live) in CDE? Have the most specific tag live. Same with
> > that thread stuff.
> 
> The motivation is to allow an unprivileged user the ability to
> configure the trust hierarchy in a way that otherwise wouldn't be
> possible for a given cgroup hierarchy.  For example given a cookie'd
> hierarchy such as:
> 
>       A
>    /  |  |   \
> B  C  D  E
> 
> the user might only want subsets of {B, C, D, E} to share.  For
> instance, the user might only want {B,C} and {D, E} to share.  One way
> to solve this would be to allow the user to write the group cookie
> directly.  However, this interface would need to be restricted to
> privileged users, since otherwise the cookie could be configured to
> share with any arbitrary cgroup.  The purpose of the 'color' field is
> to expose a portion of the cookie that can be modified by a
> non-privileged user in order to achieve this sharing goal.
> 
> If this doesn't seem like a useful case, I'm happy to drop this patch
> from the series to unblock it.

Well, the traditional cgroup way of doing that would be to:

         A
	/ \
       T1 T2
      / \
     B   C

And tag T1 if you want B,C to share.

So me the color thing reads like an end-run around the cgroup hierarchy.