Re: [PATCH RFC 06/22] drivers: base: Use present CPUs in GENERIC_CPU_DEVICES

[Shaoqin Huang <shahuang@redhat.com>]

Hi Russell,

On 11/7/23 18:29, Russell King (Oracle) wrote:
> From: James Morse <james.morse@arm.com>
> 
> Three of the five ACPI architectures create sysfs entries using
> register_cpu() for present CPUs, whereas arm64, riscv and all
> GENERIC_CPU_DEVICES do this for possible CPUs.
> 
> Registering a CPU is what causes them to show up in sysfs.
> 
> It makes very little sense to register all possible CPUs. Registering
> a CPU is what triggers the udev notifications allowing user-space to
> react to newly added CPUs.
> 
> To allow all five ACPI architectures to use GENERIC_CPU_DEVICES, change
> it to use for_each_present_cpu(). Making the ACPI architectures use
> GENERIC_CPU_DEVICES is a pre-requisite step to centralise their
> cpu_register() logic, before moving it into the ACPI processor driver.
> When ACPI is disabled this work would be done by
> cpu_dev_register_generic().

What do you actually mean about when ACPI is disabled this work would be 
done by cpu_dev_register_generic()? Is the work means register the cpu?

I'm not quite understand that, and how about when ACPI is enabled, which 
function do this work?

> 
> Of the ACPI architectures that register possible CPUs, arm64 and riscv
> do not support making possible CPUs present as they use the weak 'always
> fails' version of arch_register_cpu().
> 
> Only two of the eight architectures that use GENERIC_CPU_DEVICES have a
> distinction between present and possible CPUs.
> 
> The following architectures use GENERIC_CPU_DEVICES but are not SMP,
> so possible == present:
>   * m68k
>   * microblaze
>   * nios2
> 
> The following architectures use GENERIC_CPU_DEVICES and consider
> possible == present:
>   * csky: setup_smp()
>   * processor_probe() sets possible for all CPUs and present for all CPUs
>     except the boot cpu, which will have been done by
>     init/main.c::start_kernel().
> 
> um appears to be a subarchitecture of x86.
> 
> The remaining architecture using GENERIC_CPU_DEVICES are:
>   * openrisc and hexagon:
>     where smp_init_cpus() makes all CPUs < NR_CPUS possible,
>     whereas smp_prepare_cpus() only makes CPUs < setup_max_cpus present.
> 
> After this change, openrisc and hexagon systems that use the max_cpus
> command line argument would not see the other CPUs present in sysfs.
> This should not be a problem as these CPUs can't bre brought online as
                                              ^ nit: can't be
> _cpu_up() checks cpu_present().
> 
> After this change, only CPUs which are present appear in sysfs.
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

Thanks,
Shaoqin

> ---
>   drivers/base/cpu.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index 9ea22e165acd..34b48f660b6b 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -533,7 +533,7 @@ static void __init cpu_dev_register_generic(void)
>   #ifdef CONFIG_GENERIC_CPU_DEVICES
>   	int i;
>   
> -	for_each_possible_cpu(i) {
> +	for_each_present_cpu(i) {
>   		if (register_cpu(&per_cpu(cpu_devices, i), i))
>   			panic("Failed to register CPU device");
>   	}