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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
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