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

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Message-ID: <20190124112153.pwdsbxjynq6chmvl@e110439-lin>
Date:   Thu, 24 Jan 2019 11:21:53 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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 23-Jan 19:59, Peter Zijlstra wrote:
> 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

Yes, understand, don't want to enter a language war :)

> 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.

Right, that pattern is not "class specific" true and I should have not
used that term to begin with.

What I was trying to point out is that all the calls above directly
affect the current scheduling decision and "requires" a
dequeue/enqueue pattern.

When a task-specific uclamp value is changed for a task, instead, a
dequeue/enqueue is not needed. As long as we are doing a lazy update,
that sounds just like not necessary overhead.

However, there could still be value in keeping code consistent and if
you prefer it this way what should I do?

---8<---
    __sched_setscheduler()
        ...
        if (policy < 0)
            policy = oldpolicy = p->policy;
        ...
        if (unlikely(policy == p->policy)) {
            ...
            if (uclamp_changed())         // Force dequeue/enqueue
                goto change;
        }
    change:
        ...

        if (queued)
	    dequeue_task(rq, p, queue_flags);
	if (running)
	    put_prev_task(rq, p);

        __setscheduler_uclamp();
	__setscheduler(rq, p, attr, pi);

	if (queued)
	    enqueue_task(rq, p, queue_flags);
	if (running)
	    set_curr_task(rq, p);
        ...
---8<---

Could be something like that ok with you?

Not sure about side-effects on p->prio(): for CFS seems to be reset to
NORMAL in this case :/

-- 
#include <best/regards.h>

Patrick Bellasi