linux-kernel - Re: [PATCH v6 04/11] cpufreq/schedutil: use rt utilization tracking

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Message-ID: <20180622075817.GC17737@localhost.localdomain>
Date:   Fri, 22 Jun 2018 09:58:17 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, rjw@...ysocki.net,
        dietmar.eggemann@....com, Morten.Rasmussen@....com,
        viresh.kumar@...aro.org, valentin.schneider@....com,
        patrick.bellasi@....com, joel@...lfernandes.org,
        daniel.lezcano@...aro.org, quentin.perret@....com,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v6 04/11] cpufreq/schedutil: use rt utilization tracking

On 21/06/18 20:45, Peter Zijlstra wrote:
> On Fri, Jun 08, 2018 at 02:09:47PM +0200, Vincent Guittot wrote:
> >  static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu)
> >  {
> >  	struct rq *rq = cpu_rq(sg_cpu->cpu);
> > +	unsigned long util;
> >  
> >  	if (rq->rt.rt_nr_running)
> >  		return sg_cpu->max;
> >  
> > +	util = sg_cpu->util_dl;
> > +	util += sg_cpu->util_cfs;
> > +	util += sg_cpu->util_rt;
> > +
> >  	/*
> >  	 * Utilization required by DEADLINE must always be granted while, for
> >  	 * FAIR, we use blocked utilization of IDLE CPUs as a mechanism to
> > @@ -197,7 +204,7 @@ static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu)
> >  	 * 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.
> >  	 */
> > -	return min(sg_cpu->max, (sg_cpu->util_dl + sg_cpu->util_cfs));
> > +	return min(sg_cpu->max, util);
> >  }
> 
> So this (and the dl etc. equivalents) result in exactly the problems
> complained about last time, no?
> 
> What I proposed was something along the lines of:
> 
> 	util = 1024 * sg_cpu->util_cfs;
> 	util /= (1024 - (sg_cpu->util_rt + sg_cpu->util_dl + ...));
> 
> 	return min(sg_cpu->max, util + sg_cpu->bw_dl);
> 
> Where we, instead of directly adding the various util signals.
> 
> I now see an email from Quentin asking if these things are not in fact
> the same, but no, they are not. The difference is that the above only
> affects the CFS signal and will re-normalize the utilization of an
> 'always' running task back to 1 by compensating for the stolen capacity.
> 
> But it will not, like these here patches, affect the OPP selection of
> other classes. If there is no CFS utilization (or very little), then the
> renormalization will not matter, and the existing DL bandwidth
> compuation will be unaffected.

IIUC, even with very little CFS utilization, the final OPP selection
will still be "inflated" w.r.t. bw_dl in case util_dl is big (like for a
DL task with a big period and not so big runtime). But I guess that's
OK, since we agreed that such DL tasks should be the exception anyway.