Re: [PATCH] LoongArch: Do not create sysfs control file for io master CPUs

[Tiezhu Yang]

On 10/08/2022 05:51 PM, Tiezhu Yang wrote:
> On 10/08/2022 05:27 PM, WANG Xuerui wrote:
> > On 2022/10/8 16:59, Tiezhu Yang wrote:
> > > Now io master CPUs are not hotpluggable on LoongArch, in the current
> > > code,
> > > only /sys/devices/system/cpu/cpu0/online is not created, let us set the
> > > hotpluggable field of all the io master CPUs as 0, then prevent to
> > > create
> > > sysfs control file for the other io master CPUs which confuses some user
> > > space tools. This is similar with commit 9cce844abf07 ("MIPS: CPU#0 is
> > > not
> > > hotpluggable").
> > >
> > > Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
> > > ---
> > >   arch/loongarch/kernel/smp.c      |  8 --------
> > >   arch/loongarch/kernel/topology.c | 12 +++++++++++-
> > >   2 files changed, 11 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/arch/loongarch/kernel/smp.c b/arch/loongarch/kernel/smp.c
> > > index b5fab30..ef89292 100644
> > > --- a/arch/loongarch/kernel/smp.c
> > > +++ b/arch/loongarch/kernel/smp.c
> > > @@ -240,19 +240,11 @@ void loongson3_smp_finish(void)
> > >     #ifdef CONFIG_HOTPLUG_CPU
> > >   -static bool io_master(int cpu)
> > > -{
> > > -    return test_bit(cpu, &loongson_sysconf.cores_io_master);
> > > -}
> > > -
> > >   int loongson3_cpu_disable(void)
> > >   {
> > >       unsigned long flags;
> > >       unsigned int cpu = smp_processor_id();
> > >   -    if (io_master(cpu))
> > > -        return -EBUSY;
> > > -
> >
> > Could this get invoked from somewhere other than the sysfs entries that
> > "confuse user-space tools", e.g. from somewhere else in kernel land? If
> > so (or if we can't rule out the possibility) keeping the guard here
> > might prove more prudent.

takedown_cpu()  kernel/cpu.c
take_cpu_down()  kernel/cpu.c
__cpu_disable()  arch/loongarch/include/asm/smp.h
loongson3_cpu_disable()  arch/loongarch/kernel/smp.c

So I think you are right, keeping the guard here might prove more 
prudent, then it is better move io_master() to a header file that
can be used in smp.c and topology.c.

Let us wait for more review comments, thank you.

> If c->hotpluggable is 0, it will not generate a control file in sysfs
> for this CPU, for example:
>
> [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 it here, just remove the code.
>
> This was done in commit cbab54d9c2b2 ("MIPS: No need to check CPU 0 in
> {loongson3,bmips,octeon}_cpu_disable()").
>
> > >   #ifdef CONFIG_NUMA
> > >       numa_remove_cpu(cpu);
> > >   #endif
> > > diff --git a/arch/loongarch/kernel/topology.c
> > > b/arch/loongarch/kernel/topology.c
> > > index ab1a75c..7e7a77f 100644
> > > --- a/arch/loongarch/kernel/topology.c
> > > +++ b/arch/loongarch/kernel/topology.c
> > > @@ -5,6 +5,7 @@
> > >   #include <linux/node.h>
> > >   #include <linux/nodemask.h>
> > >   #include <linux/percpu.h>
> > > +#include <asm/bootinfo.h>
> > >     static DEFINE_PER_CPU(struct cpu, cpu_devices);
> > >   @@ -33,6 +34,11 @@ void arch_unregister_cpu(int cpu)
> > >   EXPORT_SYMBOL(arch_unregister_cpu);
> > >   #endif
> > >   +static bool io_master(int cpu)
> > > +{
> > > +    return test_bit(cpu, &loongson_sysconf.cores_io_master);
> > > +}
> > > +
> > >   static int __init topology_init(void)
> > >   {
> > >       int i, ret;
> > > @@ -40,7 +46,11 @@ static int __init topology_init(void)
> > >       for_each_present_cpu(i) {
> > >           struct cpu *c = &per_cpu(cpu_devices, i);
> > >   -        c->hotpluggable = !!i;
> > > +        if (io_master(i))
> > > +            c->hotpluggable = 0;
> > > +        else
> > > +            c->hotpluggable = 1;
> > > +
> >
> > This is just "c->hotpluggable = !io_master(i);".
>
> Yes, I am OK either way, if it is necessary to send v2,
> please let me know.
>
> > >           ret = register_cpu(c, i);
> > >           if (ret < 0)
> > >               pr_warn("topology_init: register_cpu %d failed (%d)\n",
> > > i, ret);
> > Other changes should be okay as they are in line with the previous MIPS
> > change you referenced, but let's see what Huacai thinks.
>
> Thanks,
> Tiezhu