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

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Date:   Thu, 24 Jan 2019 13:38:14 +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 Thu, Jan 24, 2019 at 11:21:53AM +0000, Patrick Bellasi wrote:

> 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.

When that overhead is shown to be a problem, is when we'll look at that
:-)

> 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?

Yes, that's about what I was expecting.

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

That's what we need KEEP_PARAM for, right?