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From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Tue, 28 Mar 2017 11:29:31 +0200
Message-ID: <CAKfTPtCgQQU-8fdxyq+9pywAtu-DFo7ZBugjEaFxjVVxu_pqjw@mail.gmail.com>
Subject: Re: [RFD PATCH 3/5] sched/cpufreq_schedutil: make worker kthread be SCHED_DEADLINE
To: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@arm.com>, "mingo@redhat.com" <mingo@redhat.com>,
        "rjw@rjwysocki.net" <rjw@rjwysocki.net>,
        viresh kumar <viresh.kumar@linaro.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
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        Thomas Gleixner <tglx@linutronix.de>,
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        Morten Rasmussen <morten.rasmussen@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
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On 27 March 2017 at 18:50, Peter Zijlstra <peterz@infradead.org> wrote:
> On Fri, Mar 24, 2017 at 02:08:58PM +0000, Juri Lelli wrote:
>> Worker kthread needs to be able to change frequency for all other
>> threads.
>>
>> Make it special, just under STOP class.
>
> *yuck* ;-)
>
> So imagine our I2C/SPI bus is 'busy' and its mutex taken, then this
> 'soecial' task will need to boost it. Now add BWI to your thinking and
> shudder.
>
>
> On IRC broonie mentioned that:
>
>  - most PMIC operations are fire and forget (no need to wait for a
>    response).
>  - PMIC 'packets' are 'small'.
>  - SPI has the possibility to push stuff on the queue.
>
> Taken together this seems to suggest we can rework cpufreq drivers to
> function in-context, either directly push the packet on the bus if
> available, or queue it and let whoever owns it sort it without blocking.
>
> It might be possible to rework/augment I2C to also support pushing stuff
> on a queue.

But sending new voltage value to PMIC is only part of the sequence.
When cpufreq set a new opp, it does

-set new voltage
-wait for the voltage to settle down.
-set the new clock frequency

you can even have to switch to an intermediate clock source.

When such sequence is managed by the kernel, we can't easily git ride
of a kthread

>
>
> So if we can make all that work, we can do away with this horrible
> horrible kthread. Which is, IMO, a much better solution.
>
> Thoughts?