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

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On 11/7/23 20:29, Russell King (Oracle) wrote:
From: James Morse <james.morse@xxxxxxx>

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@xxxxxxx>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
Signed-off-by: Russell King (Oracle) <rmk+kernel@xxxxxxxxxxxxxxx>
---
  drivers/base/cpu.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)


Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>




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