linux-kernel - Re: [PATCH V2 1/4] cpufreq: stats: Defer stats update to cpufreq_stats_record

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Date:   Thu, 24 Sep 2020 18:09:21 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Lukasz Luba <lukasz.luba@....com>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        cristian.marussi@....com, Sudeep Holla <sudeep.holla@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 1/4] cpufreq: stats: Defer stats update to
 cpufreq_stats_record_transition()

On 24-09-20, 13:07, Rafael J. Wysocki wrote:
> On Thu, Sep 24, 2020 at 1:00 PM Lukasz Luba <lukasz.luba@....com> wrote:
> > On 9/24/20 11:24 AM, Rafael J. Wysocki wrote:
> > > On Thu, Sep 24, 2020 at 11:25 AM Lukasz Luba <lukasz.luba@....com> wrote:

> > >> I wonder if we could just drop the reset feature. Is there a tool
> > >> which uses this file? The 'reset' sysfs would probably have to stay
> > >> forever, but an empty implementation is not an option?
> > >
> > > Well, having an empty sysfs attr would be a bit ugly, but the
> > > implementation of it could be simplified.
> > >
> > >> The documentation states:
> > >> 'This can be useful for evaluating system behaviour under different
> > >> governors without the need for a reboot.'
> > >> With the scenario of fast-switch this resetting complicates the
> > >> implementation and the justification of having it just for experiments
> > >> avoiding reboot is IMO weak. The real production code would have to pay
> > >> extra cycles every time. Also, we would probably not experiment with
> > >> cpufreq different governors, since the SchedUtil is considered the best
> > >> option.
> > >
> > > It would still be good to have a way to test it against the other
> > > available options, though.
> > >
> >
> > Experimenting with different governors would still be possible, just
> > the user-space would have to take a snapshot of the stats when switching
> > to a new governor. Then the values presented in the stats would just
> > need to be calculated in this user tool against the snapshot.
> >
> > The resetting is also not that bad, since nowadays more components
> > maintain some kind of local statistics/history (scheduler, thermal).
> > I would recommend to reset the whole system and repeat the same tests
> > with different governor, just to be sure that everything starts from
> > similar state (utilization, temperature, other devfreq devices
> > frequencies etc).
> 
> Well, if everyone agrees on removing the reset feature, let's drop the
> sysfs attr too, as it would be useless going forward.
> 
> Admittedly, I don't have a strong opinion and since intel_pstate
> doesn't use a frequency table, this is not relevant for systems using
> that driver anyway.

I added this file sometime back as it made my life a lot easier while testing
some scheduler related changes and see how they affect cpufreq updates. IMO this
is a useful feature and we don't really need to get rid of it.

Lets see where the discussion goes about the feedback you gave.

-- 
viresh