Date:   Wed, 8 Jun 2022 20:13:07 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
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Subject: Re: [PATCH 02/36] x86/idle: Replace x86_idle with a static_call

On Wed, Jun 8, 2022 at 4:47 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> Typical boot time setup; no need to suffer an indirect call for that.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

> ---
>  arch/x86/kernel/process.c |   50 +++++++++++++++++++++++++---------------------
>  1 file changed, 28 insertions(+), 22 deletions(-)
>
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -24,6 +24,7 @@
>  #include <linux/cpuidle.h>
>  #include <linux/acpi.h>
>  #include <linux/elf-randomize.h>
> +#include <linux/static_call.h>
>  #include <trace/events/power.h>
>  #include <linux/hw_breakpoint.h>
>  #include <asm/cpu.h>
> @@ -692,7 +693,23 @@ void __switch_to_xtra(struct task_struct
>  unsigned long boot_option_idle_override = IDLE_NO_OVERRIDE;
>  EXPORT_SYMBOL(boot_option_idle_override);
>
> -static void (*x86_idle)(void);
> +/*
> + * We use this if we don't have any better idle routine..
> + */
> +void __cpuidle default_idle(void)
> +{
> +       raw_safe_halt();
> +}
> +#if defined(CONFIG_APM_MODULE) || defined(CONFIG_HALTPOLL_CPUIDLE_MODULE)
> +EXPORT_SYMBOL(default_idle);
> +#endif
> +
> +DEFINE_STATIC_CALL_NULL(x86_idle, default_idle);
> +
> +static bool x86_idle_set(void)
> +{
> +       return !!static_call_query(x86_idle);
> +}
>
>  #ifndef CONFIG_SMP
>  static inline void play_dead(void)
> @@ -715,28 +732,17 @@ void arch_cpu_idle_dead(void)
>  /*
>   * Called from the generic idle code.
>   */
> -void arch_cpu_idle(void)
> -{
> -       x86_idle();
> -}
> -
> -/*
> - * We use this if we don't have any better idle routine..
> - */
> -void __cpuidle default_idle(void)
> +void __cpuidle arch_cpu_idle(void)
>  {
> -       raw_safe_halt();
> +       static_call(x86_idle)();
>  }
> -#if defined(CONFIG_APM_MODULE) || defined(CONFIG_HALTPOLL_CPUIDLE_MODULE)
> -EXPORT_SYMBOL(default_idle);
> -#endif
>
>  #ifdef CONFIG_XEN
>  bool xen_set_default_idle(void)
>  {
> -       bool ret = !!x86_idle;
> +       bool ret = x86_idle_set();
>
> -       x86_idle = default_idle;
> +       static_call_update(x86_idle, default_idle);
>
>         return ret;
>  }
> @@ -859,20 +865,20 @@ void select_idle_routine(const struct cp
>         if (boot_option_idle_override == IDLE_POLL && smp_num_siblings > 1)
>                 pr_warn_once("WARNING: polling idle and HT enabled, performance may degrade\n");
>  #endif
> -       if (x86_idle || boot_option_idle_override == IDLE_POLL)
> +       if (x86_idle_set() || boot_option_idle_override == IDLE_POLL)
>                 return;
>
>         if (boot_cpu_has_bug(X86_BUG_AMD_E400)) {
>                 pr_info("using AMD E400 aware idle routine\n");
> -               x86_idle = amd_e400_idle;
> +               static_call_update(x86_idle, amd_e400_idle);
>         } else if (prefer_mwait_c1_over_halt(c)) {
>                 pr_info("using mwait in idle threads\n");
> -               x86_idle = mwait_idle;
> +               static_call_update(x86_idle, mwait_idle);
>         } else if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST)) {
>                 pr_info("using TDX aware idle routine\n");
> -               x86_idle = tdx_safe_halt;
> +               static_call_update(x86_idle, tdx_safe_halt);
>         } else
> -               x86_idle = default_idle;
> +               static_call_update(x86_idle, default_idle);
>  }
>
>  void amd_e400_c1e_apic_setup(void)
> @@ -925,7 +931,7 @@ static int __init idle_setup(char *str)
>                  * To continue to load the CPU idle driver, don't touch
>                  * the boot_option_idle_override.
>                  */
> -               x86_idle = default_idle;
> +               static_call_update(x86_idle, default_idle);
>                 boot_option_idle_override = IDLE_HALT;
>         } else if (!strcmp(str, "nomwait")) {
>                 /*
>
>

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