Re: Crashes in arm qemu emulations due to 'cpufreq: governor: Replace timers with utilization ...'

[Guenter Roeck]

On 02/15/2016 11:01 AM, Tony Lindgren wrote:
> * Rafael J. Wysocki <rafael@xxxxxxxxxx> [160215 10:44]:
> > On Mon, Feb 15, 2016 at 6:05 PM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > > Rafael,
> >
> > Hi,
> >
> > Thanks for the report!
> >
> > > I see crashes in various arm qemu tests due to 'cpufreq: governor: Replace
> > > timers with utilization update callbacks' with next-20160215. An example
> > > crash log and bisect results are attached below.
> > >
> > > Please let me know if there is anything I can do to help tracking down
> > > the problem.
> >
> > It looks like we've uncovered some nastiness in the arch ARM code (see below).
> >
> > [cut]
> >
> > > [    1.340000] Unable to handle kernel NULL pointer dereference at virtual address 00000000
> > > [    1.340000] pgd = c0204000
> > > [    1.340000] [00000000] *pgd=00000000
> > > [    1.340000] Internal error: Oops: 80000005 [#1] SMP ARM
> > > [    1.340000] Modules linked in:
> > > [    1.340000] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.5.0-rc4-next-20160215 #1
> > > [    1.340000] Hardware name: Generic OMAP3-GP (Flattened Device Tree)
> > > [    1.340000] task: cb060000 ti: cb05a000 task.ti: cb05a000
> > > [    1.340000] PC is at 0x0
> > > [    1.340000] LR is at arch_send_call_function_single_ipi+0x34/0x38
> >
> > Since this is ARM, arch_send_call_function_single_ipi() looks like this:
> >
> > void arch_send_call_function_single_ipi(int cpu)
> > {
> >           smp_cross_call(cpumask_of(cpu), IPI_CALL_FUNC_SINGLE);
> > }
> >
> > so I'm not sure how the NULL pointer deref is possible even.
> >
> > The only thing coming to mind would be that cpumask_of(cpu) triggers
> > this, but I'm not sure how exactly that can happen.
> >
> > I need help from somebody who knows how this low-level stuff works on ARM.
>
> That's not even an SMP machine? I suspect a bunch of out of the
> 65 boot failures here are related to this:
>
> https://kernelci.org/boot/all/job/next/kernel/next-20160215/
>
> The SMP ones seem to fail with some regulator issues?

There is another problem, introduced with 6a0712f6f199e ("PM / OPP: Add
dev_pm_opp_set_rate()"). The kernelci boot log for next-20160212:omap3-overo-tobi
and others experience that problem.

Essentially, the code now assumes that a CPU clock always has a voltage
regulator attached to it, which is not correct. I sent out a patch to fix
that problem a minute ago.

Guenter