linux-kernel - Re: [PATCH v5 03/18] ACPI: processor: Register deferred CPUs from acpi_processor_get

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Message-ID: <CAJZ5v0gNvy2e=hOGQQ2kLpnrDr8=QGBax-E5odEJ=7BA8qW-9A@mail.gmail.com>
Date: Fri, 12 Apr 2024 20:30:40 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: linux-pm@...r.kernel.org, loongarch@...ts.linux.dev, 
	linux-acpi@...r.kernel.org, linux-arch@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, x86@...nel.org, Russell King <linux@...linux.org.uk>, 
	"Rafael J . Wysocki" <rafael@...nel.org>, Miguel Luis <miguel.luis@...cle.com>, 
	James Morse <james.morse@....com>, Salil Mehta <salil.mehta@...wei.com>, 
	Jean-Philippe Brucker <jean-philippe@...aro.org>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, linuxarm@...wei.com, justin.he@....com, 
	jianyong.wu@....com
Subject: Re: [PATCH v5 03/18] ACPI: processor: Register deferred CPUs from acpi_processor_get_info()

On Fri, Apr 12, 2024 at 4:38 PM Jonathan Cameron
<Jonathan.Cameron@...wei.com> wrote:
>
> From: James Morse <james.morse@....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 never be successfully
> called.
>
> 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@....com>
> Reviewed-by: Gavin Shan <gshan@...hat.com>
> Tested-by: Miguel Luis <miguel.luis@...cle.com>
> Tested-by: Vishnu Pajjuri <vishnu@...amperecomputing.com>
> Tested-by: Jianyong Wu <jianyong.wu@....com>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>
> Co-developed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Signed-off-by: Joanthan Cameron <Jonathan.Cameron@...wei.com>
> ---
> 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, 12 insertions(+)
>
> diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
> index 93e029403d05..c78398cdd060 100644
> --- a/drivers/acpi/acpi_processor.c
> +++ b/drivers/acpi/acpi_processor.c
> @@ -317,6 +317,18 @@ static int acpi_processor_get_info(struct acpi_device *device)
>
>         c = &per_cpu(cpu_devices, pr->id);
>         ACPI_COMPANION_SET(&c->dev, device);
> +       /*
> +        * Register CPUs that are present. get_cpu_device() is used to skip
> +        * duplicate CPU descriptions from firmware.
> +        */
> +       if (!invalid_logical_cpuid(pr->id) && cpu_present(pr->id) &&
> +           !get_cpu_device(pr->id)) {
> +               int ret = arch_register_cpu(pr->id);
> +
> +               if (ret)
> +                       return ret;
> +       }
> +
>         /*
>          *  Extra Processor objects may be enumerated on MP systems with
>          *  less than the max # of CPUs. They should be ignored _iff
> --

I am still unsure why there need to be two paths calling
arch_register_cpu() in acpi_processor_get_info().

Just below the comment partially pulled into the patch context above,
there is this code:

if (invalid_logical_cpuid(pr->id) || !cpu_present(pr->id)) {
         int ret = acpi_processor_hotadd_init(pr);

        if (ret)
                return ret;
}

For the sake of the argument, fold acpi_processor_hotadd_init() into
it and drop the redundant _STA check from it:

if (invalid_logical_cpuid(pr->id) || !cpu_present(pr->id)) {
        if (invalid_phys_cpuid(pr->phys_id))
                return -ENODEV;

        cpu_maps_update_begin();
        cpus_write_lock();

       ret = acpi_map_cpu(pr->handle, pr->phys_id, pr->acpi_id, &pr->id);
       if (ret) {
                cpus_write_unlock();
                cpu_maps_update_done();
                return ret;
       }
       ret = arch_register_cpu(pr->id);
       if (ret) {
                acpi_unmap_cpu(pr->id);

                cpus_write_unlock();
                cpu_maps_update_done();
                return ret;
       }
      pr_info("CPU%d has been hot-added\n", pr->id);
      pr->flags.need_hotplug_init = 1;

      cpus_write_unlock();
      cpu_maps_update_done();
}

so I'm not sure why this cannot be combined with the new code.

Say acpi_map_cpu) / acpi_unmap_cpu() are turned into arch calls.
What's the difference then?  The locking, which should be fine if I'm
not mistaken and need_hotplug_init that needs to be set if this code
runs after the processor driver has loaded AFAICS.