linux-kernel - Re: [PATCH RFC 06/22] drivers: base: Use present CPUs in GENERIC_CPU

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Date:   Mon, 13 Nov 2023 10:37:56 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
        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,
        linux-riscv@...ts.infradead.org, kvmarm@...ts.linux.dev,
        x86@...nel.org, linux-csky@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-parisc@...r.kernel.org
Cc:     Salil Mehta <salil.mehta@...wei.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        jianyong.wu@....com, justin.he@....com,
        James Morse <james.morse@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>
Subject: Re: [PATCH RFC 06/22] drivers: base: Use present CPUs in
 GENERIC_CPU_DEVICES

On 11/7/23 20:29, Russell King (Oracle) wrote:
> From: James Morse <james.morse@....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().
> 
> 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
> _cpu_up() checks cpu_present().
> 
> After this change, only CPUs which are present appear in sysfs.
> 
> Signed-off-by: James Morse <james.morse@....com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>
> ---
>   drivers/base/cpu.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 

Reviewed-by: Gavin Shan <gshan@...hat.com>