Re: [PATCH] cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable()

[Jinjie Ruan <ruanjinjie@huawei.com>]

On 4/18/2026 11:05 PM, Catalin Marinas wrote:
> On Sat, Apr 18, 2026 at 12:55:22PM +0100, Catalin Marinas wrote:
>> On Fri, Apr 17, 2026 at 03:55:34PM +0800, Jinjie Ruan wrote:
>>> When booting with `maxcpus` greater than the number of present CPUs (e.g.,
>>> QEMU -smp cpus=4,maxcpus=8), some CPUs are marked as 'present' but have not
>>> yet been registered via register_cpu(). Consequently, the per-cpu device
>>> objects for these CPUs are not yet initialized.
> [...]
>> Another option would have been to avoid marking such CPUs present but I
>> think this will break other things. Yet another option is to register
>> all CPU devices even if they never come up (like maxcpus greater than
>> actual CPUs).
> 
> Something like below, untested (and I don't claim I properly understand
> this code; just lots of tokens used trying to make sense of it ;))
> 
> ------------------------8<-------------------------
> diff --git a/arch/arm64/kernel/acpi.c b/arch/arm64/kernel/acpi.c
> index a9d884fd1d00..4c0a5ed906ea 100644
> --- a/arch/arm64/kernel/acpi.c
> +++ b/arch/arm64/kernel/acpi.c
> @@ -448,12 +448,14 @@ int acpi_map_cpu(acpi_handle handle, phys_cpuid_t physid, u32 apci_id,
>  		return *pcpu;
>  	}
>  
> +	set_cpu_present(*pcpu, true);
>  	return 0;
>  }
>  EXPORT_SYMBOL(acpi_map_cpu);
>  
>  int acpi_unmap_cpu(int cpu)
>  {
> +	set_cpu_present(cpu, false);

This logic, where we set 'present' in acpi_map_cpu() and clear it in
acpi_unmap_cpu(), seems to align with how x86 does it.

acpi_map_cpu()
  -> topology_hotplug_apic()
     -> topo_set_cpuids()
        -> set_cpu_present(cpu, true)

acpi_unmap_cpu()
  -> topology_hotunplug_apic(cpu)
     -> set_cpu_present(cpu, false)

Should we consider moving the setting/clearing of the 'present' bit into
the generic ACPI code (e.g., within the success path of acpi_map_cpu)?
This would ensure consistency across architectures and prevent new
implementations from missing these critical state updates.

>  	return 0;
>  }
>  EXPORT_SYMBOL(acpi_unmap_cpu);
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 1aa324104afb..751a74d997e1 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -510,8 +510,10 @@ int arch_register_cpu(int cpu)
>  	struct cpu *c = &per_cpu(cpu_devices, cpu);
>  
>  	if (!acpi_disabled && !acpi_handle &&
> -	    IS_ENABLED(CONFIG_ACPI_HOTPLUG_CPU))
> +	    IS_ENABLED(CONFIG_ACPI_HOTPLUG_CPU)) {
> +		set_cpu_present(cpu, false);
>  		return -EPROBE_DEFER;
> +	}
>  
>  #ifdef CONFIG_ACPI_HOTPLUG_CPU
>  	/* For now block anything that looks like physical CPU Hotplug */
>