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From:   Josh Don <joshdon@google.com>
Date:   Thu, 3 Dec 2020 16:20:22 -0800
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Subject: Re: [PATCH -tip 22/32] sched: Split the cookie and setup per-task
 cookie on fork
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
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On Tue, Dec 1, 2020 at 11:55 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Then disallow sharing a task cookie when the tasks are in different
> cgroups or disallow cgroup movement when they share a cookie.

Yes, we could restrict task cookie sharing to tasks that are in the
same cgroup. Then the cookie easily just becomes a single value;
either the task cookie or group cookie.

The advantage of the approach with the cookie struct is that it is
easily extensible, and allows for trust models that don't conform
exactly to the cgroup hierarchy (ie. our discussion on cookie color).
The overhead of the approach seems tolerable, given that updates to a
task's cookie are not in fast paths (ie. prctl, setting cgroup cookie,
sched_move_task).  Are you more concerned with the added complexity of
maintaining the RB tree, refcounts, etc?