Re: [PATCH v6 06/16] ACPI: processor: Register deferred CPUs from acpi_processor_get_info()

["Rafael J. Wysocki" <rafael@kernel.org>]

On Wed, Apr 17, 2024 at 5:38 PM Jonathan Cameron
<Jonathan.Cameron@huawei.com> wrote:
>
> On Wed, 17 Apr 2024 16:03:51 +0100
> Salil Mehta <salil.mehta@huawei.com> wrote:
>
> > >  From: Jonathan Cameron <jonathan.cameron@huawei.com>
> > >  Sent: Wednesday, April 17, 2024 2:19 PM
> > >
> > >  From: James Morse <james.morse@arm.com>
> > >
> > >  The arm64 specific arch_register_cpu() call may defer CPU registration until
> > >  the ACPI interpreter is available and the _STA method can be evaluated.
> > >
> > >  If this occurs, then a second attempt is made in acpi_processor_get_info().
> > >  Note that the arm64 specific call has not yet been added so for now this will
> > >  be called for the original hotplug case.
> > >
> > >  For architectures that do not defer until the ACPI Processor driver loads
> > >  (e.g. x86), for initially present CPUs there will already be a CPU device. If
> > >  present do not try to register again.
> > >
> > >  Systems can still be booted with 'acpi=off', or not include an ACPI
> > >  description at all as in these cases arch_register_cpu() will not have
> > >  deferred registration when first called.
> > >
> > >  This moves the CPU register logic back to a subsys_initcall(), while the
> > >  memory nodes will have been registered earlier.
> > >  Note this is where the call was prior to the cleanup series so there should be
> > >  no side effects of moving it back again for this specific case.
> > >
> > >  [PATCH 00/21] Initial cleanups for vCPU HP.
> > >  https://lore.kernel.org/all/ZVyz%2FVe5pPu8AWoA@shell.armlinux.org.uk/
> > >
> > >  e.g. 5b95f94c3b9f ("x86/topology: Switch over to GENERIC_CPU_DEVICES")
> > >
> > >  Signed-off-by: James Morse <james.morse@arm.com>
> > >  Reviewed-by: Gavin Shan <gshan@redhat.com>
> > >  Tested-by: Miguel Luis <miguel.luis@oracle.com>
> > >  Tested-by: Vishnu Pajjuri <vishnu@os.amperecomputing.com>
> > >  Tested-by: Jianyong Wu <jianyong.wu@arm.com>
> > >  Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
> > >  Co-developed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > >  Signed-off-by: Joanthan Cameron <Jonathan.Cameron@huawei.com>
> > >  ---
> > >  v6: Squash the two paths for conventional CPU Hotplug and arm64
> > >      vCPU HP.
> > >  v5: Update commit message to make it clear this is moving the
> > >      init back to where it was until very recently.
> > >
> > >      No longer change the condition in the earlier registration point
> > >      as that will be handled by the arm64 registration routine
> > >      deferring until called again here.
> > >  ---
> > >   drivers/acpi/acpi_processor.c | 12 +++++++++++-
> > >   1 file changed, 11 insertions(+), 1 deletion(-)
> > >
> > >  diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
> > >  index 7ecb13775d7f..0cac77961020 100644
> > >  --- a/drivers/acpi/acpi_processor.c
> > >  +++ b/drivers/acpi/acpi_processor.c
> > >  @@ -356,8 +356,18 @@ static int acpi_processor_get_info(struct
> > >  acpi_device *device)
> > >      *
> > >      *  NOTE: Even if the processor has a cpuid, it may not be present
> > >      *  because cpuid <-> apicid mapping is persistent now.
> > >  +   *
> > >  +   *  Note this allows 3 flows, it is up to the arch_register_cpu()
> > >  +   *  call to reject any that are not supported on a given architecture.
> > >  +   *  A) CPU becomes present.
> > >  +   *  B) Previously invalid logical CPU ID (Same as becoming present)
> > >  +   *  C) CPU already present and now being enabled (and wasn't
> > >  registered
> > >  +   *     early on an arch that doesn't defer to here)
> > >      */
> > >  -  if (invalid_logical_cpuid(pr->id) || !cpu_present(pr->id)) {
> > >  +  if ((!invalid_logical_cpuid(pr->id) && cpu_present(pr->id) &&
> > >  +       !get_cpu_device(pr->id)) ||
> > >  +      invalid_logical_cpuid(pr->id) ||
> > >  +      !cpu_present(pr->id)) {
> >
> >
> Hi Salil,
>
> Thanks for quick review!
>
> > Logic is clear but it is ugly. We should turn them into macro or inline.
>
> You've found the 'ugly' in this approach vs keeping them separate.
>
> For this version I wanted to keep it clear that indeed this condition
> is a complex mess of different things (and to let people compare
> it easily with the two paths in v5 to convinced themselves this
> is the same)
>
> It's also a little tricky to do, so will need some thought.
>
> I don't think a simple acpi_cpu_is_hotplug() condition is useful
> as it just moves the complexity away from where a reader is looking
> and it would only be used in this one case.
>
> It doesn't separate well into finer grained subconditions because
> (C) is a messy case of the vCPU HP case and a not done
> something else earlier.  The disadvantage of only deferring for
> arm64 and not other architectures.
>
> The best I can quickly come up with is something like this:
> #define acpi_cpu_not_present(cpu) \
>         (invalid_logical_cpuid(cpu) || !cpu_present(cpu))
> #define acpi_cpu_not_enabled(cpu) \
>         (!invalid_logical_cpuid(cpu) || cpu_present(cpu))
>
>         if ((apci_cpu_not_enabled(pr->id) && !get_cpu_device(pr->id) ||
>             acpi_cpu_not_present(pr->id))
>
> Which would still need the same amount of documentation. The
> code still isn't enough for me to immediately be able to see
> what is going on.
>
> So maybe worth it... I'm not sure.  Rafael, you get to keep this
> fun, what would you prefer?

I would use a static inline function returning bool to carry out these
checks with comments explaining the different cases in which 'true'
needs to be returned.