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

[Rafael J. Wysocki]

On Mon, Feb 15, 2016 at 8:23 PM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> On Mon, Feb 15, 2016 at 07:03:33PM +0000, Marc Zyngier wrote:
>> On 15/02/16 18:54, Rafael J. Wysocki wrote:
>> > That would explain it, thanks.
>> >
>> > So it looks like we should always use irq_work_queue() on UP even if
>> > CONFIG_SMP is set, shouldn't we?
>>
>> Something like that, yes. CONFIG_SMP is not an indication of an SMP
>> system anymore (we've even dropped the config option on arm64).
>>
>> Hopefully num_possible_cpus() is reliable enough to let you do the right
>> thing...
>
> CONFIG_SMP just says whether to include support for SMP.  It doesn't
> mandate running on a SMP system. :)
>
> I've been looking around the usages of irq_work_queue_on in kernel/
> in -rc4, and some places seem to check for "this CPU":
>
>         /*
>          * It is possible that a restart caused this CPU to be
>          * chosen again. Don't bother with an IPI, just see if we
>          * have more to push.
>          */
>         if (unlikely(cpu == rq->cpu))
>                 goto again;
>
>         /* Try the next RT overloaded CPU */
>         irq_work_queue_on(&rt_rq->push_work, cpu);
>
> I'm not sure about tell_cpu_to_push().
>
> It's also called via tick_nohz_full_kick_cpu(), and the core scheduler
> avoids calling this for the current CPU:
>
>         if (tick_nohz_full_cpu(cpu)) {
>                 if (cpu != smp_processor_id() ||
>                     tick_nohz_tick_stopped())
>                         tick_nohz_full_kick_cpu(cpu);
>
> I'm not sure about add_nr_running() in kernel/sched/sched.h - I think
> that _could_ be a problem even without Rafael's cpufreq change.
>
> So... the question is what do we do with irq_work_queue_on() in general
> when called on non-SMP systems.

I guess it might fall back to arch_irq_work_raise() when asked to
queue on the same CPU, so long as that will always do the right thing
(ie. actually queue on the same one).

Thanks,
Rafael