linux-kernel - Re: [PATCH 4/5] cpufreq: schedutil: map raw required frequency to CPU-supported frequency

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Date:	Thu, 19 May 2016 12:35:47 -0700
From:	Steve Muckle <steve.muckle@...aro.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 4/5] cpufreq: schedutil: map raw required frequency to
 CPU-supported frequency

On Thu, May 19, 2016 at 01:37:40AM +0200, Rafael J. Wysocki wrote:
> On Mon, May 9, 2016 at 11:20 PM, Steve Muckle <steve.muckle@...aro.org> wrote:
> > The mechanisms for remote CPU updates and slow-path frequency
> > transitions are relatively expensive - the former is an IPI while the
> > latter requires waking up a thread to do work. These activities should
> > be avoided if they are not necessary. To that end, calculate the
> > actual target-supported frequency required by the new utilization
> > value in schedutil. If it is the same as the previously requested
> > frequency then there is no need to continue with the update.
> 
> Unless the max/min limits changed in the meantime, right?

Right, I'll amend the commit text. The functionality is correct AFAICS.

> >
> > Signed-off-by: Steve Muckle <smuckle@...aro.org>
> > ---
> >  kernel/sched/cpufreq_schedutil.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 6cb2ecc204ec..e185075fcb5c 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -153,14 +153,26 @@ static void sugov_update_commit(struct sugov_cpu *sg_cpu, int cpu, u64 time,
> >   * next_freq = C * curr_freq * util_raw / max
> >   *
> >   * Take C = 1.25 for the frequency tipping point at (util / max) = 0.8.
> > + *
> > + * The lowest target-supported frequency which is equal or greater than the raw
> > + * next_freq (as calculated above) is returned, or the CPU's max_freq if such
> > + * a target-supported frequency does not exist.
> >   */
> >  static unsigned int get_next_freq(struct cpufreq_policy *policy,
> >                                   unsigned long util, unsigned long max)
> >  {
> > +       struct cpufreq_frequency_table *entry;
> >         unsigned int freq = arch_scale_freq_invariant() ?
> >                                 policy->cpuinfo.max_freq : policy->cur;
> > +       unsigned int target_freq = UINT_MAX;
> > +
> > +       freq = (freq + (freq >> 2)) * util / max;
> > +
> > +       cpufreq_for_each_valid_entry(entry, policy->freq_table)
> > +               if (entry->frequency >= freq && entry->frequency < target_freq)
> > +                       target_freq = entry->frequency;
> 
> Please don't assume that every driver will have a frequency table.
> That may not be the case in the future (and I'm not even sure about
> the existing CPPC driver for that matter).

For platforms without a frequency table I guess we can just continue
with the current behavior, passing in the raw calculated frequency. I'll
make this change.

At some point I imagine those platforms will want to somehow achieve
similar behavior to avoid very small transitions that do not result in
real benefit. Maybe some sort of threshold % in the schedutil down the
road.

thanks,
Steve