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

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Date:   Tue, 28 Mar 2017 12:20:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Juri Lelli <juri.lelli@....com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        viresh kumar <viresh.kumar@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        luca.abeni@...tannapisa.it, claudio@...dence.eu.com,
        tommaso.cucinotta@...tannapisa.it, bristot@...hat.com,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Andres Oportus <andresoportus@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Patrick Bellasi <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 Tue, Mar 28, 2017 at 11:29:31AM +0200, Vincent Guittot wrote:
> On 27 March 2017 at 18:50, Peter Zijlstra <peterz@...radead.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

That stinks :-(

The whole blocking and bandwidth inheritance stuff gives me nightmares.
Makes all this stuff almost impossible to analyse.

Esp. if the bus (SPI/I2C) is shared with some other 'expensive' device
like a DSP or MMC flash crud.