linux-kernel - Re: [PATCH v6 05/16] sched/core: uclamp: Update CPU's refcount on clamp changes

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Date:   Wed, 23 Jan 2019 19:59:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v6 05/16] sched/core: uclamp: Update CPU's refcount on
 clamp changes

On Wed, Jan 23, 2019 at 02:14:26PM +0000, Patrick Bellasi wrote:

> > > Consider also that the uclamp_task_update_active() added by this patch
> > > not only has lower overhead but it will be use also by cgroups where
> > > we want to force update all the tasks on a cgroup's clamp change.
> > 
> > I haven't gotten that far; but I would prefer not to have two different
> > 'change' paths in __sched_setscheduler().
> 
> Yes, I agree that two paths in __sched_setscheduler() could be
> confusing. Still we have to consider that here we are adding
> "not class specific" attributes.

But that change thing is not class specific; the whole:


	rq = task_rq_lock(p, &rf);
	queued = task_on_rq_queued(p);
	running = task_current(rq, p);
	if (queued)
		dequeue_task(rq, p, queue_flags);
	if (running)
		put_prev_task(rq, p);


	/* @p is in it's invariant state; frob it's state */


	if (queued)
		enqueue_task(rq, p, queue_flags);
	if (running)
		set_curr_task(rq, p);
	task_rq_unlock(rq, p, &rf);


pattern is all over the place; it is just because C sucks that that
isn't more explicitly shared (do_set_cpus_allowed(), rt_mutex_setprio(),
set_user_nice(), __sched_setscheduler(), sched_setnuma(),
sched_move_task()).

This is _the_ pattern for changing state and is not class specific at
all.