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From:   Josh Don <joshdon@google.com>
Date:   Thu, 3 Dec 2020 16:51:42 -0800
Message-ID: <CABk29Ns-QWGV+XpN8TJ5CL50jhrpqKAhfoOfaPSCjiFq7S7j1Q@mail.gmail.com>
Subject: Re: [PATCH -tip 26/32] sched: Add a second-level tag for nested
 CGroup usecase
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>, mingo@kernel.org,
        torvalds@linux-foundation.org, fweisbec@gmail.com,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>, Phil Auld <pauld@redhat.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>, vineeth@bitbyteword.org,
        Chen Yu <yu.c.chen@intel.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Agata Gruza <agata.gruza@intel.com>,
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        Tom Lendacky <thomas.lendacky@amd.com>,
        Aubrey Li <aubrey.li@linux.intel.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
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        Oleg Rombakh <olegrom@google.com>, Tejun Heo <tj@kernel.org>
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On Wed, Dec 2, 2020 at 12:02 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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.

Restructuring the cgroup resource hierarchy to incorporate the trust
domains is not necessarily trivial (as is the case for us).  I agree
though that that would be the ideal correct solution from the cgroup
hierarchy perspective.