Re: [PATCH 04/12] ia64: Replace cpu_down with freeze_secondary_cpus

Qais Yousef
2019-11-19T23:19:18Z

On 11/19/19 23:59, Thomas Gleixner wrote:
> On Tue, 19 Nov 2019, Qais Yousef wrote:
> > On 11/19/19 23:21, Thomas Gleixner wrote:
> > > On Wed, 30 Oct 2019, Qais Yousef wrote:
> > > > void machine_shutdown(void)
> > > > {
> > > > #ifdef CONFIG_HOTPLUG_CPU
> > > > - int cpu;
> > > > -
> > > > - for_each_online_cpu(cpu) {
> > > > - if (cpu != smp_processor_id())
> > > > - cpu_down(cpu);
> > > > - }
> > > > + /* TODO: Can we use disable_nonboot_cpus()? */
> > > > + freeze_secondary_cpus(smp_processor_id());
> > >
> > > freeze_secondary_cpus() is only available for CONFIG_PM_SLEEP_SMP=y and
> > > disable_nonboot_cpus() is a NOOP for CONFIG_PM_SLEEP_SMP=n :)
> >
> > I thought I replied to this :-(
> >
> > My plan was to simply make freeze_secondary_cpus() available and protected by
> > CONFIG_SMP only instead.
> >
> > Good plan?
>
> No. freeze_secondary_cpus() is really for hibernation. Look at the exit
> conditions there.

Hmm do you mean the pm_wakeup_pending() abort?

In arm64 we machine_shutdown() calls disable_nonboot_cpus(), which in turn
a wrapper around freeze_secondary_cpus() with 0 passed as an argument.

IIUC this means arm64 could fail to offline all CPUs on machine_shutdown(),
correct?

>
> So you really want a seperate function which depends on CONFIG_HOTPLUG_CPU
> and provides an inline stub for the CONFIG_HOTPLUG_CPU=n case.
>
> But I have a hard time to see how that stuff works at all on
> ia64:
>
> cpu_down() might sleep, i.e. it must be called from preemptible
> context. smp_processor_id() is invalid from preemtible context.
>
> As this is obviously broken and ia64 is in keep compile mode only, it
> should just go away.

If arm64 is doing the wrong thing, then we need a new function anyway.

Thanks

--
Qais Yousef