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

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Date:   Sat, 8 Oct 2022 22:58:30 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc:     WANG Xuerui <kernel@...0n.name>, loongarch@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] LoongArch: Do not create sysfs control file for io master CPUs

On Sat, Oct 8, 2022 at 6:44 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
>
>
> 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@...ngson.cn>
> >>> ---
> >>>   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.
Agree, please send V2, thanks.


Huacai
>
> 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@...ux loongson]# cat /sys/devices/system/cpu/cpu0/online
> > cat: /sys/devices/system/cpu/cpu0/online: No such file or directory
> > [root@...ux 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
>