linux-kernel - Re: [PATCH v3 2/3] x86/smp native_play_dead: Prefer cpuidle_play_dead() over mwait_play

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Message-ID: <20241112114743.GQ22801@noisy.programming.kicks-ass.net>
Date: Tue, 12 Nov 2024 12:47:43 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	rafael.j.wysocki@...el.com, len.brown@...el.com,
	artem.bityutskiy@...ux.intel.com, dave.hansen@...ux.intel.com
Subject: Re: [PATCH v3 2/3] x86/smp native_play_dead: Prefer
 cpuidle_play_dead() over mwait_play_dead()

On Fri, Nov 08, 2024 at 01:29:08PM +0100, Patryk Wlazlyn wrote:
> The generic implementation, based on cpuid leaf 0x5, for looking up the
> mwait hint for the deepest cstate, depends on them to be continuous in
> range [0, NUM_SUBSTATES-1]. While that is correct on most Intel x86
> platforms, it is not architectural and may not result in reaching the
> most optimized idle state on some of them.
> 
> Prefer cpuidle_play_dead() over the generic mwait_play_dead() loop and
> fallback to the later in case of missing enter_dead() handler.
> 
> Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>
> ---
>  arch/x86/kernel/smpboot.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 44c40781bad6..721bb931181c 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1416,9 +1416,9 @@ void native_play_dead(void)
>  	play_dead_common();
>  	tboot_shutdown(TB_SHUTDOWN_WFS);
>  
> -	mwait_play_dead();
>  	if (cpuidle_play_dead())
> -		hlt_play_dead();
> +		mwait_play_dead();
> +	hlt_play_dead();
>  }

Yeah, I don't think so. we don't want to accidentally hit
acpi_idle_play_dead().