linux-kernel - Re: [PATCH V2 5/7] thermal/drivers/cpu_cooling: Add idle cooling device documentation

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Message-ID: <20180308085949.GB17761@amd>
Date:   Thu, 8 Mar 2018 09:59:49 +0100
From:   Pavel Machek <pavel@....cz>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     edubezval@...il.com, kevin.wangtao@...aro.org, leo.yan@...aro.org,
        vincent.guittot@...aro.org, amit.kachhap@...il.com,
        linux-kernel@...r.kernel.org, javi.merino@...nel.org,
        rui.zhang@...el.com, daniel.thompson@...aro.org,
        linux-pm@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH V2 5/7] thermal/drivers/cpu_cooling: Add idle cooling
 device documentation

Hi!

> >> +Under certain circumstances, the SoC reaches a temperature exceeding
> >> +the allocated power budget or the maximum temperature limit. The
> > 
> > I don't understand. Power budget is in W, temperature is in
> > kelvin. Temperature can't exceed power budget AFAICT.
> 
> Yes, it is badly worded. Is the following better ?
> 
> "
> Under certain circumstances a SoC can reach the maximum temperature
> limit or is unable to stabilize the temperature around a temperature
> control.
> 
> When the SoC has to stabilize the temperature, the kernel can act on a
> cooling device to mitigate the dissipated power.
> 
> When the maximum temperature is reached and to prevent a catastrophic
> situation a radical decision must be taken to reduce the temperature
> under the critical threshold, that impacts the performance.
> 
> "

Actually... if hardware is expected to protect itself, I'd tone it
down. No need to be all catastrophic and critical... But yes, better.

> > Critical here, critical there. I have trouble following
> > it. Theoretically hardware should protect itself, because you don't
> > want kernel bug to damage your CPU?
> 
> There are several levels of protection. The first level is mitigating
> the temperature from the kernel, then in the temperature sensor a reset
> line will trigger the reboot of the CPUs. Usually it is a register where
> you write the maximum temperature, from the driver itself. I never tried
> to write 1000°C in this register and see if I can burn the board.
> 
> I know some boards have another level of thermal protection in the
> hardware itself and some other don't.
> 
> In any case, from a kernel point of view, it is a critical situation as
> we are about to hard reboot the system and in this case it is preferable
> to drop drastically the performance but give the opportunity to the
> system to run in a degraded mode.

Agreed you want to keep going. In ACPI world, we shutdown when
critical trip point is reached, so this is somehow confusing.

> >> +Solutions:
> >> +----------
> >> +
> >> +If we can remove the static and the dynamic leakage for a specific
> >> +duration in a controlled period, the SoC temperature will
> >> +decrease. Acting at the idle state duration or the idle cycle
> > 
> > "should" decrease? If you are in bad environment..
> 
> No, it will decrease in any case because of the static leakage drop. The
> bad environment will impact the speed of this decrease.

I meant... if ambient temperature is 105C, there's not much you can do
to cool system down :-).

> >> +Idle Injection:
> >> +---------------
> >> +
> >> +The base concept of the idle injection is to force the CPU to go to an
> >> +idle state for a specified time each control cycle, it provides
> >> +another way to control CPU power and heat in addition to
> >> +cpufreq. Ideally, if all CPUs of a cluster inject idle synchronously,
> >> +this cluster can get into the deepest idle state and achieve minimum
> >> +power consumption, but that will also increase system response latency
> >> +if we inject less than cpuidle latency.
> > 
> > I don't understand last sentence.
> 
> Is it better ?
> 
> "Ideally, if all CPUs, belonging to the same cluster, inject their idle
> cycle synchronously, the cluster can reach its power down state with a
> minimum power consumption and static leakage drop. However, these idle
> cycles injection will add extra latencies as the CPUs will have to
> wakeup from a deep sleep state."

Extra comma "CPUs , belonging". But yes, better.

> Thanks!

You are welcome. Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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