linux-kernel - Re: [PATCH] cpufreq: dt: Set default policy->transition_delay

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Message-ID: <20170627042041.GA29665@vireshk-i7>
Date:   Tue, 27 Jun 2017 09:50:41 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Leo Yan <leo.yan@...aro.org>,
        Brendan Jackman <brendan.jackman@....com>,
        linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, Amit Kucheria <amit.kucheria@...aro.org>
Subject: Re: [PATCH] cpufreq: dt: Set default policy->transition_delay_ns

On 27-06-17, 02:15, Rafael J. Wysocki wrote:
> On Monday, May 22, 2017 04:57:27 PM Viresh Kumar wrote:
> > On 22-05-17, 19:17, Leo Yan wrote:
> > > This afternoon Amit pointed me for this patch, should fix as below?
> > > Otherwise it seems directly assign the same value from unit 'ns' to
> > > 'us' but without any value conversion.
> > > 
> > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > > index 76877a6..dcc90fc 100644
> > > --- a/kernel/sched/cpufreq_schedutil.c
> > > +++ b/kernel/sched/cpufreq_schedutil.c
> > > @@ -538,7 +538,7 @@ static int sugov_init(struct cpufreq_policy *policy)
> > >                 unsigned int lat;
> > >  
> > >                 tunables->rate_limit_us = LATENCY_MULTIPLIER;
> > > -               lat = policy->cpuinfo.transition_latency / NSEC_PER_USEC;

I think the above line is just fine and the below one is incorrect, as
we wanted to convert transition latency to usec here (i.e. in the
units of rate_limit_us).

> > > +               lat = policy->cpuinfo.transition_latency / NSEC_PER_MSEC;
> > >                 if (lat)
> > >                         tunables->rate_limit_us *= lat;
> > >         }
> > 
> > I will let Rafael comment in as well. NSEC_PER_USEC is used in the
> > earlier governors as well (ondemand/conservative) in exactly the same
> > way as schedutil is using.
> 
> The reason why it is used by schedutil is because the other governors used it
> that way.  IOW, doesn't matter. :-)

But I feel the value of LATENCY_MULTIPLIER (1000) is way too high. It currently
says that if freq-switching takes time X, then we should wait for 999X time
before we change the freq again.

Perhaps LATENCY_MULTIPLIER should be just 10 or 20 here. For a platform with
transition_latency 500 us, rate_limit_us comes to 500 ms. Which is absurd. We
ideally want it to be around 10-20 ms here. And compared to other ARM platforms,
500 us transition_latency is very low. It normally is around 1-3 ms for ARM32
platforms.

@Rafael: Will it be fine to lower down the value of LATENCY_MULTIPLIER?

-- 
viresh