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

[Juri Lelli]

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.