Re: [x86/signal] 3aac3ebea0: will-it-scale.per_thread_ops -11.9% regression

[Dave Hansen]

On 12/8/21 10:00 AM, Bae, Chang Seok wrote:
> diff --git a/kernel/signal.c b/kernel/signal.c
> index a629b11bf3e0..8194d2f38bf1 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -4224,6 +4224,11 @@ int restore_altstack(const stack_t __user *uss)
>         stack_t new;
>         if (copy_from_user(&new, uss, sizeof(stack_t)))
>                 return -EFAULT;
> +       if (current->sas_ss_sp == (unsigned long) new.ss_sp &&
> +           current->sas_ss_size == new.ss_size &&
> +           current->sas_ss_flags == new.ss_flags)
> +               return 0;
> +
>         (void)do_sigaltstack(&new, NULL, current_user_stack_pointer(),
>                              MINSIGSTKSZ);
>         /* squash all but EFAULT for now */

This seems like a generally good optimization that could go in
do_sigaltstack() itself, no?

Either way, it seems like 0day botched this a bit.  '3aac3ebea0' wasn't
the actual culprit, it was the patch before.