Re: [PATCH RFC v4 02/15] ACPI: processor: Register all CPUs from acpi_processor_get_info()

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Wed, 10 Apr 2024 15:28:18 +0200
"Rafael J. Wysocki" <rafael@kernel.org> wrote:

> On Wed, Apr 10, 2024 at 2:43 PM Jonathan Cameron
> <Jonathan.Cameron@huawei.com> wrote:
> >  
> > > >  
> > > > > diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> > > > > index 47de0f140ba6..13d052bf13f4 100644
> > > > > --- a/drivers/base/cpu.c
> > > > > +++ b/drivers/base/cpu.c
> > > > > @@ -553,7 +553,11 @@ static void __init cpu_dev_register_generic(void)
> > > > >  {
> > > > >         int i, ret;
> > > > >
> > > > > -       if (!IS_ENABLED(CONFIG_GENERIC_CPU_DEVICES))
> > > > > +       /*
> > > > > +        * When ACPI is enabled, CPUs are registered via
> > > > > +        * acpi_processor_get_info().
> > > > > +        */
> > > > > +       if (!IS_ENABLED(CONFIG_GENERIC_CPU_DEVICES) || !acpi_disabled)
> > > > >                 return;  
> > > >
> > > > Honestly, this looks like a quick hack to me and it absolutely
> > > > requires an ACK from the x86 maintainers to go anywhere.  
> > > Will address this separately.
> > >  
> >
> > So do people prefer this hack, or something along lines of the following?
> >
> > static int __init cpu_dev_register_generic(void)
> > {
> >         int i, ret = 0;
> >
> >         for_each_online_cpu(i) {
> >                 if (!get_cpu_device(i)) {
> >                         ret = arch_register_cpu(i);
> >                         if (ret)
> >                                 pr_warn("register_cpu %d failed (%d)\n", i, ret);
> >                 }
> >         }
> >         //Probably just eat the error.
> >         return 0;
> > }
> > subsys_initcall_sync(cpu_dev_register_generic);  
> 
> I would prefer something like the above.
> 
> I actually thought that arch_register_cpu() might return something
> like -EPROBE_DEFER when it cannot determine whether or not the CPU is
> really available.

Ok. That would end up looking much more like the original code I think.
So we wouldn't have this late registration at all, or keep it for DT
on arm64?  I'm not sure that's a clean solution though leaves
the x86 path alone.

If we get rid of this catch all, solution would be to move the
!acpi_disabled check into the arm64 version of arch_cpu_register()
because we would only want the delayed registration path to be
used on ACPI cases where the question of CPU availability can't
yet be resolved.

> 
> Then, the ACPI processor enumeration path may take care of registering
> CPU that have not been registered so far and in the more-or-less the
> same way regardless of the architecture (modulo some arch-specific
> stuff).

If I understand correctly, in acpi_processor_get_info() we'd end up
with a similar check on whether it was already registered (the x86 path)
or had be deferred (arm64 / acpi).
 
> 
> In the end, it should be possible to avoid changing the behavior of
> x86 and loongarch in this series.

Possible, yes, but result if I understand correctly is we end up with
very different flows and replication of functionality between the
early registration and the late one. I'm fine with that if you prefer it!

> 
> > Which may look familiar at it's effectively patch 3 from v3 which was dealing
> > with CPUs missing from DSDT (something we think doesn't happen).
> >
> > It might be possible to elide the arch_register_cpu() in
> > make_present() but that will mean we use different flows in this patch set
> > for the hotplug and initially present cases which is a bit messy.
> >
> > I've tested this lightly on arm64 and x86 ACPI + DT booting and it "seems" fine.  
> 
> Sounds promising.

Possibly not that relevant though if proposal is to drop this approach. :(
At least I now have test setups!

Jonathan
> 
> Thanks!