Re: [PATCH] MIPS: No need to check CPU 0 in {loongson3,bmips,octeon}_cpu_disable()

[Tiezhu Yang]

On 12/15/2020 11:26 AM, liwei (GF) wrote:
> Hi,
>
> On 2020/11/25 18:11, Tiezhu Yang wrote:
> > After commit 9cce844abf07 ("MIPS: CPU#0 is not hotpluggable"),
> Why CPU#0 is not hotpluggable on MIPS? Does that unrealizable?

The early discussion is here:
https://patchwork.kernel.org/project/linux-mips/patch/1594896024-16624-1-git-send-email-chenhc@xxxxxxxxxx/

As Jiaxun said, it seems that this is a software limitation.
I think the initial aim of commit 9cce844abf07 ("MIPS: CPU#0 is not 
hotpluggable")
is to solve ltp test failure.

> > c->hotpluggable is 0 for CPU 0 and it will not generate a control
> > file in sysfs for this CPU:
> >
> > [root@linux loongson]# cat /sys/devices/system/cpu/cpu0/online
> > cat: /sys/devices/system/cpu/cpu0/online: No such file or directory
> > [root@linux loongson]# echo 0 > /sys/devices/system/cpu/cpu0/online
> > bash: /sys/devices/system/cpu/cpu0/online: Permission denied
> >
> > So no need to check CPU 0 in {loongson3,bmips,octeon}_cpu_disable(),
> missing cps_cpu_disable()?

Yes, thank you, let me send a patch to do it.

Thanks,
Tiezhu

> Thanks,
> Wei
>
> > just remove them.
> >
> > Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
> > ---
> >   arch/mips/cavium-octeon/smp.c | 3 ---
> >   arch/mips/kernel/smp-bmips.c  | 3 ---
> >   arch/mips/loongson64/smp.c    | 3 ---
> >   3 files changed, 9 deletions(-)
> >
> > diff --git a/arch/mips/cavium-octeon/smp.c b/arch/mips/cavium-octeon/smp.c
> > index 076db9a..66ce552 100644
> > --- a/arch/mips/cavium-octeon/smp.c
> > +++ b/arch/mips/cavium-octeon/smp.c
> > @@ -290,9 +290,6 @@ static int octeon_cpu_disable(void)
> >   {
> >   	unsigned int cpu = smp_processor_id();
> >   
> > -	if (cpu == 0)
> > -		return -EBUSY;
> > -
> >   	if (!octeon_bootloader_entry_addr)
> >   		return -ENOTSUPP;
> >   
> > diff --git a/arch/mips/kernel/smp-bmips.c b/arch/mips/kernel/smp-bmips.c
> > index 1dbfb5a..359b176 100644
> > --- a/arch/mips/kernel/smp-bmips.c
> > +++ b/arch/mips/kernel/smp-bmips.c
> > @@ -362,9 +362,6 @@ static int bmips_cpu_disable(void)
> >   {
> >   	unsigned int cpu = smp_processor_id();
> >   
> > -	if (cpu == 0)
> > -		return -EBUSY;
> > -
> >   	pr_info("SMP: CPU%d is offline\n", cpu);
> >   
> >   	set_cpu_online(cpu, false);
> > diff --git a/arch/mips/loongson64/smp.c b/arch/mips/loongson64/smp.c
> > index aa0cd72..b8c1fc3 100644
> > --- a/arch/mips/loongson64/smp.c
> > +++ b/arch/mips/loongson64/smp.c
> > @@ -544,9 +544,6 @@ static int loongson3_cpu_disable(void)
> >   	unsigned long flags;
> >   	unsigned int cpu = smp_processor_id();
> >   
> > -	if (cpu == 0)
> > -		return -EBUSY;
> > -
> >   	set_cpu_online(cpu, false);
> >   	calculate_cpu_foreign_map();
> >   	local_irq_save(flags);