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

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Message-ID: <20170327165011.dpip23cyolnleas6@hirez.programming.kicks-ass.net>
Date:   Mon, 27 Mar 2017 18:50:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@....com>
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 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.

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?