diff mbox series

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

Message ID E1r0JLB-00CTwy-7y@rmk-PC.armlinux.org.uk (mailing list archive)
State Handled Elsewhere
Headers show
Series Initial cleanups for vCPU hotplug | expand

Commit Message

Russell King (Oracle) Nov. 7, 2023, 10:29 a.m. UTC 
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().

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@arm.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
---
 drivers/base/cpu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Shaoqin Huang Nov. 9, 2023, 10:09 a.m. UTC | #1 
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");
>   	}
Russell King (Oracle) Nov. 9, 2023, 10:29 a.m. UTC | #2 
On Thu, Nov 09, 2023 at 06:09:32PM +0800, Shaoqin Huang wrote:
> 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

Firstly, please note that "you" is not appropriate here. This is James'
commit message, not mine.

> done by cpu_dev_register_generic()? Is the work means register the cpu?

When ACPI is disabled _and_ CONFIG_GENERIC_CPU_DEVICES is enabled, then
cpu_dev_register_generic() will call arch_register_cpu() for each present
CPU after this commit, rather than for each _possible_ CPU (which is the
actual code change here.)

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

This is what happens later in the series.

"drivers: base: Allow parts of GENERIC_CPU_DEVICES to be overridden"
adds a test for CONFIG_GENERIC_CPU_DEVICES, so this will only be used
with architectures using GENERIC_CPU_DEVICES. Then in:

"ACPI: processor: Register all CPUs from acpi_processor_get_info()"
which is not part of this series, this adds a call to arch_register_cpu()
in the ACPI code, and disables this path via a test for !acpi_disabled.

Essentially, this path gets used to register the present CPUs when
firmware (ACPI) isn't going to be registering the present CPUs.

I've changed this to:

"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 register_cpu() logic, before moving it into the
ACPI processor driver. When we add support for register CPUs from ACPI
in a later patch, we will avoid registering CPUs in this path."

which I hope makes it clearer.

> > 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

Thanks, I'll fix that.
Shaoqin Huang Nov. 9, 2023, 10:59 a.m. UTC | #3 
On 11/9/23 18:29, Russell King (Oracle) wrote:
> On Thu, Nov 09, 2023 at 06:09:32PM +0800, Shaoqin Huang wrote:
>> 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
> 
> Firstly, please note that "you" is not appropriate here. This is James'
> commit message, not mine.
> 

Oh, Sorry for that.

>> done by cpu_dev_register_generic()? Is the work means register the cpu?
> 
> When ACPI is disabled _and_ CONFIG_GENERIC_CPU_DEVICES is enabled, then
> cpu_dev_register_generic() will call arch_register_cpu() for each present
> CPU after this commit, rather than for each _possible_ CPU (which is the
> actual code change here.)
> 
>> I'm not quite understand that, and how about when ACPI is enabled, which
>> function do this work?
> 
> This is what happens later in the series.
> 
> "drivers: base: Allow parts of GENERIC_CPU_DEVICES to be overridden"
> adds a test for CONFIG_GENERIC_CPU_DEVICES, so this will only be used
> with architectures using GENERIC_CPU_DEVICES. Then in:
> 
> "ACPI: processor: Register all CPUs from acpi_processor_get_info()"
> which is not part of this series, this adds a call to arch_register_cpu()
> in the ACPI code, and disables this path via a test for !acpi_disabled.
> 
> Essentially, this path gets used to register the present CPUs when
> firmware (ACPI) isn't going to be registering the present CPUs.
> 
> I've changed this to:
> 
> "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 register_cpu() logic, before moving it into the
> ACPI processor driver. When we add support for register CPUs from ACPI
> in a later patch, we will avoid registering CPUs in this path."
> 
> which I hope makes it clearer.
> 

Thanks for your great explanation. Change commit message to this makes 
me understand well.

Thanks,
Shaoqin

>>> 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
> 
> Thanks, I'll fix that.
>
Gavin Shan Nov. 13, 2023, 12:37 a.m. UTC | #4 
On 11/7/23 20:29, 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().
> 
> 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@arm.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
> ---
>   drivers/base/cpu.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 

Reviewed-by: Gavin Shan <gshan@redhat.com>
diff mbox series

Patch

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");
 	}