linux-kernel - Re: [PATCH 1/7] sched/core: uclamp: add CPU clamp groups accounting

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Date:   Fri, 13 Apr 2018 14:44:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH 1/7] sched/core: uclamp: add CPU clamp groups accounting

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.