linux-kernel - Re: [PATCH V2 6/7] thermal/drivers/cpu_cooling: Introduce the cpu idle cooling driver

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Message-ID: <20180327122836.GD10639@localhost.localdomain>
Date:   Tue, 27 Mar 2018 14:28:36 +0200
From:   Juri Lelli <juri.lelli@...il.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Leo Yan <leo.yan@...aro.org>, edubezval@...il.com,
        kevin.wangtao@...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,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH V2 6/7] thermal/drivers/cpu_cooling: Introduce the cpu
 idle cooling driver

Hi Daniel,

On 27/03/18 12:26, Daniel Lezcano wrote:
> On 27/03/2018 04:03, Leo Yan wrote:
> > Hi Daniel,
> > 
> > On Wed, Feb 21, 2018 at 04:29:27PM +0100, Daniel Lezcano wrote:
> >> The cpu idle cooling driver performs synchronized idle injection across all
> >> cpus belonging to the same cluster and offers a new method to cool down a SoC.
> >>
> >> Each cluster has its own idle cooling device, each core has its own idle
> >> injection thread, each idle injection thread uses play_idle to enter idle.  In
> >> order to reach the deepest idle state, each cooling device has the idle
> >> injection threads synchronized together.
> >>
> >> It has some similarity with the intel power clamp driver but it is actually
> >> designed to work on the ARM architecture via the DT with a mathematical proof
> >> with the power model which comes with the Documentation.
> >>
> >> The idle injection cycle is fixed while the running cycle is variable. That
> >> allows to have control on the device reactivity for the user experience. At
> >> the mitigation point the idle threads are unparked, they play idle the
> >> specified amount of time and they schedule themselves. The last thread sets
> >> the next idle injection deadline and when the timer expires it wakes up all
> >> the threads which in turn play idle again. Meanwhile the running cycle is
> >> changed by set_cur_state.  When the mitigation ends, the threads are parked.
> >> The algorithm is self adaptive, so there is no need to handle hotplugging.
> > 
> > The idle injection threads are RT threads (FIFO) and I saw in
> > play_idle() set/clear flag PF_IDLE for it.  Will these idle injection
> > threads utilization be accounted into RT utilization?
> > 
> > If idle injection threads utilization is accounted as RT tasks
> > utilization, will this impact CPUFreq governor 'schedutil' for OPP
> > selection?
> 
> Hi Leo,
> 
> The idle injection task has a very low utilization when it is not in the
> play_idle function, basically it wakes up, sets a timer and play_idle().
> 
> Regarding the use case, the idle injection is the base brick for an
> combo cooling device with cpufreq + cpuidle. When the idle injection is
> used alone, it is because there is no cpufreq driver for the platform.
> If there is a cpufreq driver, then we should endup with the cpu cooling
> device where we have control of the OPP (and there is no idle injection
> threads) or the combo cooling device.
> 
> Except I'm missing something, the idle injection threads won't impact
> the OPP selection.

Mmm, they actually might. schedutil selects max OPP as soon as it sees
an RT thread. I fear these guys might generate unwanted spikes. Maybe
you can filter them out?

Best,

- Juri