linux-kernel - Re: [RFC PATCH v2 1/8] sched/cpufreq_schedutil: make use of DEADLINE utilization signal

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Message-ID: <20171205152424.GC15085@localhost.localdomain>
Date:   Tue, 5 Dec 2017 16:24:24 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     peterz@...radead.org, mingo@...hat.com, rjw@...ysocki.net,
        viresh.kumar@...aro.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, tglx@...utronix.de,
        vincent.guittot@...aro.org, rostedt@...dmis.org,
        luca.abeni@...tannapisa.it, claudio@...dence.eu.com,
        tommaso.cucinotta@...tannapisa.it, bristot@...hat.com,
        mathieu.poirier@...aro.org, tkjos@...roid.com, joelaf@...gle.com,
        morten.rasmussen@....com, dietmar.eggemann@....com,
        alessio.balsini@....com, Juri Lelli <juri.lelli@....com>,
        Ingo Molnar <mingo@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [RFC PATCH v2 1/8] sched/cpufreq_schedutil: make use of DEADLINE
 utilization signal

Hi,

On 05/12/17 15:09, Patrick Bellasi wrote:
> Hi Juri,
> 

[...]

> >  static void sugov_get_util(unsigned long *util, unsigned long *max, int cpu)
> >  {
> >  	struct rq *rq = cpu_rq(cpu);
> > -	unsigned long cfs_max;
> > +	unsigned long dl_util = (rq->dl.running_bw * SCHED_CAPACITY_SCALE)
> > +				>> BW_SHIFT;
> 
> What about using a pair of getter methods (e.g. cpu_util_{cfs,dl}) to
> be defined in kernel/sched/sched.h?
> 
> This would help to hide class-specific signals mangling from cpufreq.
> And here we can have something "more abstract" like:
> 
>        unsigned long util_cfs = cpu_util_cfs(rq);
>        unsigned long util_dl = cpu_util_dl(rq);

LGTM. I'll cook something for next spin.

> 
> >  
> > -	cfs_max = arch_scale_cpu_capacity(NULL, cpu);
> > +	*max = arch_scale_cpu_capacity(NULL, cpu);
> >  
> > -	*util = min(rq->cfs.avg.util_avg, cfs_max);
> > -	*max = cfs_max;
> > +	/*
> > +	 * Ideally we would like to set util_dl as min/guaranteed freq and
> > +	 * util_cfs + util_dl as requested freq. However, cpufreq is not yet
> > +	 * ready for such an interface. So, we only do the latter for now.
> > +	 */
> 
> Maybe I don't completely get the above comment, but to me it is not
> really required.
> 
> When you say that "util_dl" should be set to a min/guaranteed freq
> are you not actually talking about a DL implementation detail?
> 
> From the cpufreq standpoint instead, we should always set a capacity
> which can accommodate util_dl + util_cfs.

It's more for platforms which supports such combination of values for
frequency requests (CPPC like, AFAIU). The idea being that util_dl is
what the system has to always guarantee, but it could go up to the sum
if feasible.

> 
> We don't care about the meaning of util_dl and we should always assume
> (by default) that the signal is properly updated by the scheduling
> class... which unfortunately does not always happen for CFS.
> 
> 
> > +	*util = min(rq->cfs.avg.util_avg + dl_util, *max);
> 
> With the above proposal, here also we will have:
> 
> 	*util = min(util_cfs + util_dl, *max);

Looks cleaner.

Thanks,

- Juri