linux-kernel - Re: [RFD PATCH 3/5] sched/cpufreq_schedutil: make worker kthread be SCHED

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Date:   Mon, 27 Mar 2017 18:01:34 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, rjw@...ysocki.net, viresh.kumar@...aro.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        tglx@...utronix.de, vincent.guittot@...aro.org,
        rostedt@...dmis.org, luca.abeni@...tannapisa.it,
        claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
        bristot@...hat.com, mathieu.poirier@...aro.org, tkjos@...roid.com,
        joelaf@...gle.com, andresoportus@...gle.com,
        morten.rasmussen@....com, dietmar.eggemann@....com,
        patrick.bellasi@....com, Ingo Molnar <mingo@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [RFD PATCH 3/5] sched/cpufreq_schedutil: make worker kthread be
 SCHED_DEADLINE

On 27/03/17 18:50, Peter Zijlstra 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* ;-)
> 

Eh, I know. :/

> 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.
> 

Currently that kthread is FIFO already, so boosting still applies. Not as
bad as in the BWI case though. More thinking required.

> 
> 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.
> 
> 
> 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?

Right. This is more a schedutil (cpufreq) problem though, IMHO. Even if
I agree that what you are proposing is way more clean (and here I
actually assume it's feasible at all), I fear it will take quite some
time to get reworked.

Do we want to wait until that moment to get DEADLINE contribution
accounted for? :(

Thanks,

- Juri