linux-kernel - Re: [PATCH v1 2/4] LoongArch: Handle percpu handler address in bt

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Message-ID: <CAAhV-H4iJ=FJHHbX4BGqCb5MViwA7Tcz1jbh0pKywz5T48XHXg@mail.gmail.com>
Date: Wed, 21 Jan 2026 10:30:33 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/4] LoongArch: Handle percpu handler address in bt_address()

On Wed, Jan 21, 2026 at 9:47 AM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> On 2026/1/20 下午9:08, Tiezhu Yang wrote:
> > On 2026/1/20 下午9:01, Huacai Chen wrote:
> >> On Tue, Jan 20, 2026 at 8:57 PM Tiezhu Yang <yangtiezhu@...ngson.cn>
> >> wrote:
> >>>
> >>> On 2026/1/20 下午8:04, Huacai Chen wrote:
> >>>> Hi, Tiezhu,
> >>>>
> >>>> On Tue, Jan 20, 2026 at 5:23 PM Tiezhu Yang <yangtiezhu@...ngson.cn>
> >>>> wrote:
> >>>>>
> >>>>> After commit 4cd641a79e69 ("LoongArch: Remove unnecessary checks for
> >>>>> ORC unwinder"), the system can not boot normally under some configs,
> >>>>> there are many error messages "cannot find unwind pc at".
> >>>
> >>> ...
> >>>
> >>>>> +#if defined(CONFIG_NUMA) && !defined(CONFIG_PREEMPT_RT)
> >>>>> +       for (int cpu = 1; cpu < num_possible_cpus(); cpu++) {
> >>>> Use nr_cpu_ids instead of num_possible_cpus() can improve
> >>>> performance a little.
> >>>>
> >>>>> +               int vec_sz = sizeof(exception_handlers);
> >>>>> +
> >>>> It is better to add "if (!pcpu_handlers[cpu]) continue" here.
> >>>
> >>> If so, it can use for_each_possible_cpu(cpu) directly?
> >> It can, but I remember you want to skip CPU 0?
> >
> > Yes, that is the intention to use the for loop, but if checking
> > !pcpu_handlers[cpu], I think no need to use for loop to skip cpu
> > 0, just use for_each_possible_cpu(cpu) is more simple and direct.
>
> Handle this special case only if bt_address() returns 0 to avoid
> affecting performance for the normal address.
It is a little more complex than V1, since unwinder is not
performance-critical, I prefer the V1 logic.

Huacai

>
> Here is the draft change:
>
> ----->8-----
> diff --git a/arch/loongarch/kernel/unwind_orc.c
> b/arch/loongarch/kernel/unwind_orc.c
> index d6b3688a1ce9..1ce10701f3c3 100644
> --- a/arch/loongarch/kernel/unwind_orc.c
> +++ b/arch/loongarch/kernel/unwind_orc.c
> @@ -378,6 +378,28 @@ static inline unsigned long bt_address(unsigned
> long ra)
>          return 0;
>   }
>
> +static inline unsigned long handle_pcpu_handlers(unsigned long ra)
> +{
> +#if defined(CONFIG_NUMA) && !defined(CONFIG_PREEMPT_RT)
> +       int cpu;
> +       int vec_sz __maybe_unused;
> +
> +       for_each_possible_cpu(cpu) {
> +               if (!pcpu_handlers[cpu])
> +                       continue;
> +
> +               vec_sz = sizeof(exception_handlers);
> +
> +               if (ra >= pcpu_handlers[cpu] &&
> +                   ra < pcpu_handlers[cpu] + vec_sz) {
> +                       ra = eentry + (ra - pcpu_handlers[cpu]);
> +                       break;
> +               }
> +       }
> +#endif
> +       return ra;
> +}
> +
>   bool unwind_next_frame(struct unwind_state *state)
>   {
>          unsigned long *p, pc;
> @@ -494,8 +516,11 @@ bool unwind_next_frame(struct unwind_state *state)
>
>          state->pc = bt_address(pc);
>          if (!state->pc) {
> -               pr_err("cannot find unwind pc at %px\n", (void *)pc);
> -               goto err;
> +               state->pc = bt_address(handle_pcpu_handlers(pc));
> +               if (!state->pc) {
> +                       pr_err("cannot find unwind pc at %px\n", (void
> *)pc);
> +                       goto err;
> +               }
>          }
>
>          return true;
>
> I will test it, if it works well I will wait for more comments and
> submit v2 later.
>
> Thanks,
> Tiezhu
>