linux-kernel - Re: [PATCH v6 11/16] sched/fair: Add uclamp support to energy

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Date:   Tue, 22 Jan 2019 12:45:46 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Quentin Perret <quentin.perret@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        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 11/16] sched/fair: Add uclamp support to
 energy_compute()

On 22-Jan 12:13, Quentin Perret wrote:
> On Tuesday 15 Jan 2019 at 10:15:08 (+0000), Patrick Bellasi wrote:
> > The Energy Aware Scheduler (AES) estimates the energy impact of waking

[...]

> > +		for_each_cpu_and(cpu, pd_mask, cpu_online_mask) {
> > +			cfs_util = cpu_util_next(cpu, p, dst_cpu);
> > +
> > +			/*
> > +			 * Busy time computation: utilization clamping is not
> > +			 * required since the ratio (sum_util / cpu_capacity)
> > +			 * is already enough to scale the EM reported power
> > +			 * consumption at the (eventually clamped) cpu_capacity.
> > +			 */
> 
> Right.
> 
> > +			sum_util += schedutil_cpu_util(cpu, cfs_util, cpu_cap,
> > +						       ENERGY_UTIL, NULL);
> > +
> > +			/*
> > +			 * Performance domain frequency: utilization clamping
> > +			 * must be considered since it affects the selection
> > +			 * of the performance domain frequency.
> > +			 */
> 
> So that actually affects the way we deal with RT I think. I assume the
> idea is to say if you don't want to reflect the RT-go-to-max-freq thing
> in EAS (which is what we do now) you should set the min cap for RT to 0.
> Is that correct ?

By default configuration, RT tasks still go to max when uclamp is
enabled, since they get a util_min=1024.

If we want to save power on RT tasks, we can set a smaller util_min...
but not necessarily 0. A util_min=0 for RT tasks means to use just
cpu_util_rt() for that class.

> I'm fine with this conceptually but maybe the specific case of RT should
> be mentioned somewhere in the commit message or so ? I think it's
> important to say that clearly since this patch changes the default
> behaviour.

Default behavior for RT should not be affected. While a capping is
possible for those tasks... where do you see issues ?
Here we are just figuring out what's the capacity the task will run
at, if we will have clamped RT tasks will not be the max but: is that
a problem ?

> > +			cpu_util = schedutil_cpu_util(cpu, cfs_util, cpu_cap,
> > +						      FREQUENCY_UTIL,
> > +						      cpu == dst_cpu ? p : NULL);
> > +			max_util = max(max_util, cpu_util);
> >  		}
> >  
> >  		energy += em_pd_energy(pd->em_pd, max_util, sum_util);
> 
> Thanks,
> Quentin

-- 
#include <best/regards.h>

Patrick Bellasi