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Date:   Tue, 27 Mar 2018 15:08:08 +0200
From:   Juri Lelli <juri.lelli@gmail.com>
To:     Daniel Lezcano <daniel.lezcano@linaro.org>
Cc:     Leo Yan <leo.yan@linaro.org>, edubezval@gmail.com,
        kevin.wangtao@linaro.org, vincent.guittot@linaro.org,
        amit.kachhap@gmail.com, linux-kernel@vger.kernel.org,
        javi.merino@kernel.org, rui.zhang@intel.com,
        daniel.thompson@linaro.org, linux-pm@vger.kernel.org,
        Viresh Kumar <viresh.kumar@linaro.org>
Subject: Re: [PATCH V2 6/7] thermal/drivers/cpu_cooling: Introduce the cpu
 idle cooling driver
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On 27/03/18 14:31, Daniel Lezcano wrote:
> On 27/03/2018 14:28, Juri Lelli wrote:
> > 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?
> 
> Yes, absolutely. Leo pointed it also.
> 
> We might want to change the check at:
> 
> https://elixir.bootlin.com/linux/v4.16-rc7/source/kernel/sched/cpufreq_schedutil.c#L364
> 
> in order to ignore PF_IDLE tagged tasks.

We might yes. And also for the update_single cases, I guess.

Best,

- Juri