linux-kernel - Re: [PATCH v6 10/16] sched/core: Add uclamp_util

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Message-ID: <20190123145106.zaqb3d6l65rs5lg6@e110439-lin>
Date:   Wed, 23 Jan 2019 14:51:06 +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 10/16] sched/core: Add uclamp_util_with()

On 23-Jan 14:33, Peter Zijlstra wrote:
> On Tue, Jan 15, 2019 at 10:15:07AM +0000, Patrick Bellasi wrote:
> > +static __always_inline
> > +unsigned int uclamp_util_with(struct rq *rq, unsigned int util,
> > +			      struct task_struct *p)
> >  {
> >  	unsigned int min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
> >  	unsigned int max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
> >  
> > +	if (p) {
> > +		min_util = max(min_util, uclamp_effective_value(p, UCLAMP_MIN));
> > +		max_util = max(max_util, uclamp_effective_value(p, UCLAMP_MAX));
> > +	}
> > +
> 
> Like I think you mentioned earlier; this doesn't look right at all.

What we wanna do here is to compute what _will_ be the clamp values of
a CPU if we enqueue *p on it.

The code above starts from the current CPU clamp value and mimics what
uclamp will do in case we move the task there... which is always a max
aggregation.

> Should that not be something like:
> 
> 	lo = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
> 	hi = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
> 
> 	min_util = clamp(uclamp_effective(p, UCLAMP_MIN), lo, hi);
> 	max_util = clamp(uclamp_effective(p, UCLAMP_MAX), lo, hi);

Here you end up with a restriction of the task clamp (effective)
clamps values considering the CPU clamps... which is different.

Why do you think we should do that?... perhaps I'm missing something.

-- 
#include <best/regards.h>

Patrick Bellasi