Date: Tue, 28 Mar 2017 12:20:31 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.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>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>, luca.abeni@santannapisa.it,
        claudio@evidence.eu.com, tommaso.cucinotta@santannapisa.it,
        bristot@redhat.com, Mathieu Poirier <mathieu.poirier@linaro.org>,
        Todd Kjos <tkjos@android.com>, Joel Fernandes <joelaf@google.com>,
        Andres Oportus <andresoportus@google.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Mark Brown <broonie@kernel.org>
Subject: Re: [RFD PATCH 3/5] sched/cpufreq_schedutil: make worker kthread be
 SCHED_DEADLINE
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On Tue, Mar 28, 2017 at 11:29:31AM +0200, Vincent Guittot wrote:
> 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

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.