Re: BUG_ON(irqs_disabled()) in cpufreq.c

[Mike Turquette <mturquette@linaro.org>]

Quoting Viresh Kumar (2013-08-01 22:02:37)
> More lists cc'd
> 
> On Thu, Aug 1, 2013 at 6:08 PM, Michael Giardino
> <giardino@ece.gatech.edu> wrote:
> > Hi,
> >
> > I hope this is the right spot to ask this.
> >
> > Is there any way to change the cpu frequency using the cpufreq driver
> > from within an hrtimer callback function? I encounter a kernel panic
> > at cpufreq.c line 255 (260 in 3.9.5)

It is not just about the notifiers.

Performing a clock rate change and voltage regulator transition (DVFS)
is not a short operation on many ARM platforms. Relocking a PLL and
waiting for a regulator to step and stabilize can take well over 100
milliseconds and some of those functions might sleep.

Some ARM platform do not manage this directly and talk to a power
management micro controller, so we could probably do that with
interrupts disabled if it's just a matter of writing to some
memory-mapped registers.

I don't know how Intel does it with ACPI but I assume that they don't
block waiting for things to happen based on a recent G+ post I read[1].
I have even less of a clue how the other architectures out there do it.

So the question in general is whether we should assume that we may sleep
or block during a CPU frequency scaling operation.  The short answer
seems to be "yes" since some platforms need this today.

Perhaps CPUfreq could allow for differentiating between platforms that
need to sleep/block and those that do not?

Regards,
Mike

[1] https://plus.google.com/114657443111661859546/posts/dLn9T4ehywL

> >
> > void __cpufreq_notify_transition(struct cpufreq_policy *policy,
> > 253                 struct cpufreq_freqs *freqs, unsigned int state)
> > 254 {
> > 255         BUG_ON(irqs_disabled());
> >
> >
> > I'm assuming this is due to the cpufreq_notify_transition needs
> > interrupts to notify? Is there a way around this? It appears that
> > acpi-cpufreq.c is calling the notifier both before and after the
> > transition (CPUFREQ_PRECHANGE and CPUFREQ_POSTCHANGE).
> >
> > Are there any frequency change functions that can operate without interrupts?
> >
> > The background is this:
> >
> > For the past few days I have been trying to get to the bottom of a
> > problem involving a nonSMP governor I have written. The goal of this
> > governor is to change the frequency on a predefined schedule (in the
> > ms scale). The basic breakdown is this:
> >
> > 1. The external scheduler computes schedules and then passes them to
> > the governor (with much code appropriated from the userspace governor)
> > via an exported function.
> > 2. The governor sets the frequency, then sets a timer to call the next
> > frequency change when it goes off
> >
> > In order to do this, I was using hrtimers. These timer's callback
> > functions run with interrupts disabled.
> >
> > Michael Giardino
> > <giardino@ece.gatech.edu>
> > <michael.giardino@gmail.com>
> > --
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