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Date:   Fri, 13 Apr 2018 14:44:58 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Patrick Bellasi <patrick.bellasi@arm.com>
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>, Tejun Heo <tj@kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Paul Turner <pjt@google.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Joel Fernandes <joelaf@google.com>,
        Steve Muckle <smuckle@google.com>
Subject: Re: [PATCH 1/7] sched/core: uclamp: add CPU clamp groups accounting
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 <20180413111510.GS14248@e110439-lin>
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On Fri, Apr 13, 2018 at 12:47:45PM +0100, Patrick Bellasi wrote:
> In the past I remember some funny dance in cgroup callbacks when a
> task was terminating (like being moved in the root-rq just before
> exiting). But, as you say, if we always have the task_rq_lock we
> should be safe.

The syscall does the whole:

	task_rq_lock();
	queued = task_on_rq_queued();
	running = task_current();
	if (queued)
		dequeue_task();
	if (running)
		put_prev_task();

	/*
	 * task is in invariant state here,
	 * muck with it.
	 */

	if (queued)
		enqueue_task();
	if (running)
		set_curr_task()
	task_rq_unlock();

pattern; which because C sucks, we've not found a good template for yet.

Without the dequeue/put,enqueue/set you have to jump a few extra hoops.