linux-kernel - Re: [PATCH v2 3/3] PM / EM: Skip inefficient OPPs

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Message-ID: <20210601084725.GA223449@e120877-lin.cambridge.arm.com>
Date:   Tue, 1 Jun 2021 09:47:25 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, rjw@...ysocki.net,
        vincent.guittot@...aro.org, qperret@...gle.com,
        linux-kernel@...r.kernel.org, ionela.voinescu@....com,
        lukasz.luba@....com, dietmar.eggemann@....com
Subject: Re: [PATCH v2 3/3] PM / EM: Skip inefficient OPPs

On Fri, May 28, 2021 at 10:39:34AM +0530, Viresh Kumar wrote:
> On 25-05-21, 10:48, Peter Zijlstra wrote:
> > On Fri, May 21, 2021 at 05:54:24PM +0100, Vincent Donnefort wrote:
> > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > > index 4f09afd..5a91a2b 100644
> > > --- a/kernel/sched/cpufreq_schedutil.c
> > > +++ b/kernel/sched/cpufreq_schedutil.c
> > > @@ -10,6 +10,7 @@
> > >  
> > >  #include "sched.h"
> > >  
> > > +#include <linux/energy_model.h>
> > >  #include <linux/sched/cpufreq.h>
> > >  #include <trace/events/power.h>
> > >  
> > > @@ -153,6 +154,9 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> > >  
> > >  	freq = map_util_freq(util, freq, max);
> > >  
> > > +	/* Avoid inefficient performance states */
> > > +	freq = em_pd_get_efficient_freq(em_cpu_get(policy->cpu), freq);
> > > +
> > >  	if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
> > >  		return sg_policy->next_freq;
> > >  
> > 
> > This seems somewhat unfortunate, it adds a loop over the OPPs only to
> > then call into cpufreq to do the exact same thing again :/
> 
> And that's why I feel it needs to be done at a single place, either disable the
> OPP (which seems like a bad option based on what Lukasz and Vincent said
> earlier), or make changes in the cpufreq core itself to search for the best
> frequency (like adding another API to mark some frequencies as inefficient, and
> take that into account while selecting next freq).
> 
> There is a potential of ending up selecting the wrong frequency here because
> there are too many decision making bodies here and so corner cases.
> 
> -- 
> viresh

Hi Viresh,

Seems like no one has been really convinced about the arguments in favor of
keeping inefficiencies into EM :) Let me then give a shot with marking the OPPs
for the next version.

-- 
Vincent