linux-kernel - Re: [PATCH V3 2/4] thermal/drivers/cpu_cooling: Add idle cooling device documentation

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Date:   Wed, 4 Dec 2019 12:40:25 +0530
From:   Amit Kucheria <amit.kucheria@...aro.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Zhang Rui <rui.zhang@...el.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Eduardo Valentin <edubezval@...il.com>,
        Linux PM list <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3 2/4] thermal/drivers/cpu_cooling: Add idle cooling
 device documentation

On Wed, Dec 4, 2019 at 12:20 PM Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
>
>
> Hi Amit,
>
> thanks for the review.
>
>
> On 04/12/2019 05:24, Amit Kucheria wrote:
>
> [ ... ]
>
> >> +the CPUs will have to wakeup from a deep sleep state.
> >> +
> >> +     ^
> >> +     |
> >> +     |
> >> +     |-------       -------       -------
> >> +     |_______|_____|_______|_____|_______|___________
> >> +
> >> +      <----->
> >> +       idle  <---->
> >> +              running
> >> +
> >> +With the fixed idle injection duration, we can give a value which is
> >> +an acceptable performance drop off or latency when we reach a specific
> >> +temperature and we begin to mitigate by varying the Idle injection
> >> +period.
> >> +
> >
> > I'm not sure what it the purpose of this statement. You've described
> > how the period value starts at a maximum and is adjusted dynamically
> > below.
>
> We can have different way to inject idle periods. We can increase the
> idle duration and/or keep this duration constant but make a variation of
> the period. This statement clarify the method which is the latter
> because we want to have a constant latency per period easier to deal with.

I think I read period as duration leading to confusion. I suggest
using duty-cycle instead of period throughout this series. I think it
will improve the explanation.

The above paragraph could be rewritten as:

"We use a fixed duration of idle injection that gives an acceptable
performance penalty and a fixed latency. Mitigation can be increased
or decreased by modulating the duty cycle of the idle injection."

Perhaps you could also enhance your ascii art above to show fixed
duration idles and different duty cyles to drive home the point.

> >> +The mitigation begins with a maximum period value which decrease when
> >
> > Shouldn't the idle injection period increase to get more cooling effect?
>
> The period is the opposite of the frequency. The highest the period, the
> lowest the frequency, thus less idle cycles and lesser cooling effect.

Yeah, I definitely confused period with duration :-)

> >> +more cooling effect is requested. When the period duration is equal to
> >> +the idle duration, then we are in a situation the platform can’t
> >> +dissipate the heat enough and the mitigation fails. In this case the
> >> +situation is considered critical and there is nothing to do. The idle
> >> +injection duration must be changed by configuration and until we reach
> >> +the cooling effect, otherwise an additionnal cooling device must be
> >
> > typo: additional
> >
> >> +used or ultimately decrease the SoC performance by dropping the
> >> +highest OPP point of the SoC.
> >> +
> >> +The idle injection duration value must comply with the constraints:
> >> +
> >> +- It is lesser or equal to the latency we tolerate when the mitigation
> >
> > s/lesser/less than/
> >
> >> +  begins. It is platform dependent and will depend on the user
> >> +  experience, reactivity vs performance trade off we want. This value
> >> +  should be specified.
> >> +
> >> +- It is greater than the idle state’s target residency we want to go
> >> +  for thermal mitigation, otherwise we end up consuming more energy.
> >> +
> >> +Minimum period
> >> +--------------
> >> +
> >> +The idle injection duration being fixed, it is obvious the minimum
> >
> > Change to:
> > When the idle injection duration is fixed,
> >
>
> The idle duration is always fixed in the cpuidle cooling device, why do
> you want to add the sentence above?

Ignore for now.

Regards,
Amit