linux-kernel - Re: [PATCH 1/2][v2] cpuidle-haltpoll: Replace default_idle with arch_cpu

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Date:   Wed, 7 Dec 2022 15:37:43 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     lirongqing@...du.com
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, rafael@...nel.org,
        daniel.lezcano@...aro.org, peterz@...radead.org,
        akpm@...ux-foundation.org, tony.luck@...el.com,
        jpoimboe@...nel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH 1/2][v2] cpuidle-haltpoll: Replace default_idle with arch_cpu_idle

On Wed, Dec 7, 2022 at 1:42 PM <lirongqing@...du.com> wrote:
>
> From: Li RongQing <lirongqing@...du.com>
>
> When KVM guest has MWAIT and mwait_idle is used as default idle function,
> but once cpuidle-haltpoll is loaded, default_idle in default_enter_idle is
> used, which is using HLT, and cause a performance regression. As the commit
> aebef63cf7ff ("x86: Remove vendor checks from prefer_mwait_c1_over_halt")
> explains that mwait is preferred
>
> so replace default_idle with arch_cpu_idle which can using MWAIT
> optimization.
>
> latency of sockperf ping-pong localhost test is reduced by more 20%
> unixbench has 5% performance improvements for single core
>
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> ---
>  arch/x86/kernel/process.c          | 1 +
>  drivers/cpuidle/cpuidle-haltpoll.c | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index c21b734..303afad 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -721,6 +721,7 @@ void arch_cpu_idle(void)
>  {
>         x86_idle();
>  }
> +EXPORT_SYMBOL(arch_cpu_idle);

Why do you need this export at all?

>
>  /*
>   * We use this if we don't have any better idle routine..
> diff --git a/drivers/cpuidle/cpuidle-haltpoll.c b/drivers/cpuidle/cpuidle-haltpoll.c
> index 3a39a7f..e66df22 100644
> --- a/drivers/cpuidle/cpuidle-haltpoll.c
> +++ b/drivers/cpuidle/cpuidle-haltpoll.c
> @@ -32,7 +32,7 @@ static int default_enter_idle(struct cpuidle_device *dev,
>                 local_irq_enable();
>                 return index;
>         }
> -       default_idle();
> +       arch_cpu_idle();
>         return index;
>  }
>
> --
> 2.9.4
>