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.
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