linux-kernel - Re: [RFCv7 PATCH 02/10] cpufreq: introduce cpufreq_driver_is

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Date:	Thu, 25 Feb 2016 16:50:29 -0800
From:	Michael Turquette <mturquette@...libre.com>
To:	"Rafael J. Wysocki" <rafael@...nel.org>,
	"Steve Muckle" <steve.muckle@...aro.org>
Cc:	"Peter Zijlstra" <peterz@...radead.org>,
	"Ingo Molnar" <mingo@...hat.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Vincent Guittot" <vincent.guittot@...aro.org>,
	"Morten Rasmussen" <morten.rasmussen@....com>,
	"Dietmar Eggemann" <dietmar.eggemann@....com>,
	"Juri Lelli" <Juri.Lelli@....com>,
	"Patrick Bellasi" <patrick.bellasi@....com>,
	"Viresh Kumar" <viresh.kumar@...aro.org>
Subject: Re: [RFCv7 PATCH 02/10] cpufreq: introduce cpufreq_driver_is_slow

Quoting Rafael J. Wysocki (2016-02-22 17:31:09)
> On Tue, Feb 23, 2016 at 2:22 AM, Steve Muckle <steve.muckle@...aro.org> wrote:
> > From: Michael Turquette <mturquette@...libre.com>
> >
> > Some architectures and platforms perform CPU frequency transitions
> > through a non-blocking method, while some might block or sleep. Even
> > when frequency transitions do not block or sleep they may be very slow.
> > This distinction is important when trying to change frequency from
> > a non-interruptible context in a scheduler hot path.
> >
> > Describe this distinction with a cpufreq driver flag,
> > CPUFREQ_DRIVER_FAST. The default is to not have this flag set,
> > thus erring on the side of caution.
> >
> > cpufreq_driver_is_slow() is also introduced in this patch. Setting
> > the above flag will allow this function to return false.
> >
> > [smuckle@...aro.org: change flag/API to include drivers that are too
> >  slow for scheduler hot paths, in addition to those that block/sleep]
> >
> > Cc: Rafael J. Wysocki <rafael@...nel.org>
> > Cc: Viresh Kumar <viresh.kumar@...aro.org>
> > Signed-off-by: Michael Turquette <mturquette@...libre.com>
> > Signed-off-by: Steve Muckle <smuckle@...aro.org>
> 
> Something more sophisticated than this is needed, because one driver
> may actually be able to do "fast" switching in some cases and may not
> be able to do that in other cases.

Those drivers can set the flag dynamically when they probe based on
their ACPI tables.

> 
> For example, in the acpi-cpufreq case all depends on what's there in
> the ACPI tables.

It's all a moot point until the locking in cpufreq is changed. Until
those changes are made it is a bad idea to call cpufreq_driver_target()
from schedule() context, regardless of the underlying hardware, and all
platforms should kick that work out to the kthread.

Regards,
Mike

> 
> Thanks,
> Rafael