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

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Message-ID: <37277532.TseFq3j8HT@aspire.rjw.lan>
Date:   Mon, 27 Mar 2017 19:05:06 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Juri Lelli <juri.lelli@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
        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 Monday, March 27, 2017 06:01:34 PM Juri Lelli wrote:
> 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.

Why do you think so?

Thanks,
Rafael