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Date:   Wed, 7 Apr 2021 20:39:40 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Tejun Heo <tj@kernel.org>
Cc:     joel@joelfernandes.org, chris.hyser@oracle.com, joshdon@google.com,
        mingo@kernel.org, vincent.guittot@linaro.org,
        valentin.schneider@arm.com, mgorman@suse.de,
        linux-kernel@vger.kernel.org, tglx@linutronix.de,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Zefan Li <lizefan.x@bytedance.com>
Subject: Re: [PATCH 0/9] sched: Core scheduling interfaces
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On Tue, Apr 06, 2021 at 12:08:50PM -0400, Tejun Heo wrote:

> I see. Yeah, if we really need it, I'm not sure it fits in cgroup interface
> proper. As I wrote elsewhere, these things are usually implemented on the
> originating subsystem interface with cgroup ID as a parameter.

This would be something like:

  prctl(PR_SCHED_CORE, PR_SCHED_CORE_SHARE_FROM, cgroup-fd, PIDTYPE_CGROUP, NULL);

right? Where we assign to self the cookie from the cgroup.

The problem I see with this is that a task can trivially undo/circumvent
this by calling PR_SCHED_CORE_CLEAR on itself, at which point it can
share with system tasks again.

Also, it doesn't really transfer well to the group/tasks thing. When a
task joins a cgroup, it doesn't automagically gain the cgroup
properties. Whoever does the transition will then also have to prctl()
this, which nobody will do.