linux-kernel - Re: [PATCH v2 02/10] cpufreq: provide data for frequency-invariant load-tracking support

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Date:   Fri, 07 Jul 2017 00:38:31 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux@....linux.org.uk,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Juri Lelli <juri.lelli@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH v2 02/10] cpufreq: provide data for frequency-invariant load-tracking support

On Thursday, July 06, 2017 04:10:27 PM Viresh Kumar wrote:
> On 06-07-17, 10:49, Dietmar Eggemann wrote:
> > A frequency-invariant load-tracking solution based on cpufreq transition
> > notifier will not work for future fast frequency switching policies.
> > That is why a different solution is presented with this patch.
> > 
> > Let cpufreq call the function arch_set_freq_scale() to pass the current
> > frequency, the max supported frequency and the cpumask of the related
> > cpus to a consumer (an arch) which defines arch_set_freq_scale().
> > 
> > The consumer has to associate arch_set_freq_scale with the name of its
> > own implementation foo_set_freq_scale() to overwrite the empty standard
> > definition in drivers/cpufreq/cpufreq.c.
> > An arch could do this in one of its arch-specific header files
> > (e.g. arch/$ARCH/include/asm/topology.h) which gets included in
> > drivers/cpufreq/cpufreq.c.
> > 
> > In case arch_set_freq_scale() is not defined (and because of the
> > pr_debug() drivers/cpufreq/cpufreq.c is not compiled with -DDEBUG)
> 
> The line within () needs to be improved to convey a clear message.
> 
> > the
> > function cpufreq_set_freq_scale() gets compiled out.
> > 
> > Cc: Rafael J. Wysocki <rjw@...ysocki.net>
> > Cc: Viresh Kumar <viresh.kumar@...aro.org>
> > Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> > ---
> >  drivers/cpufreq/cpufreq.c | 26 ++++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> > 
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index 9bf97a366029..a04c5886a5ce 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -347,6 +347,28 @@ static void __cpufreq_notify_transition(struct cpufreq_policy *policy,
> >  	}
> >  }
> >  
> > +/*********************************************************************
> > + *           FREQUENCY INVARIANT CPU CAPACITY SUPPORT                *
> > + *********************************************************************/
> > +
> > +#ifndef arch_set_freq_scale
> > +static void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq,
> > +				unsigned long max_freq)
> > +{}
> > +#endif
> > +
> > +static void cpufreq_set_freq_scale(struct cpufreq_policy *policy,
> > +				   struct cpufreq_freqs *freqs)
> > +{
> > +	unsigned long cur_freq = freqs ? freqs->new : policy->cur;
> > +	unsigned long max_freq = policy->cpuinfo.max_freq;
> > +
> > +	pr_debug("cpus %*pbl cur/cur max freq %lu/%lu kHz\n",
> > +		 cpumask_pr_args(policy->related_cpus), cur_freq, max_freq);
> > +
> > +	arch_set_freq_scale(policy->related_cpus, cur_freq, max_freq);
> 
> I am not sure why all these are required to be sent here and will come back to
> it later on after going through other patches.
> 
> > +}
> > +
> >  /**
> >   * cpufreq_notify_transition - call notifier chain and adjust_jiffies
> >   * on frequency transition.
> > @@ -405,6 +427,8 @@ void cpufreq_freq_transition_begin(struct cpufreq_policy *policy,
> >  
> >  	spin_unlock(&policy->transition_lock);
> >  
> > +	cpufreq_set_freq_scale(policy, freqs);
> > +
> 
> Why do this before even changing the frequency ? We may fail while changing it.
> 
> IMHO, you should call this routine whenever we update policy->cur and that
> happens regularly in __cpufreq_notify_transition() and few other places..

There seems to be a general problem with doing this in the core with respect to
things like intel_pstate that use their own governor callbacks and don't invoke
cpufreq_freq_transition_begin() then.

Thanks,
Rafael