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: <20241113162222.GA24625@noisy.programming.kicks-ass.net>
Date: Wed, 13 Nov 2024 17:22:22 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Gautham R. Shenoy" <gautham.shenoy@....com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>, 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 Wed, Nov 13, 2024 at 05:11:38PM +0530, Gautham R. Shenoy wrote:

> How about something like this (completely untested)
> 
> ---------------------x8----------------------------------------------------
> 
> diff --git a/arch/x86/kernel/acpi/cstate.c b/arch/x86/kernel/acpi/cstate.c
> index f3ffd0a3a012..bd611771fa6c 100644
> --- a/arch/x86/kernel/acpi/cstate.c
> +++ b/arch/x86/kernel/acpi/cstate.c
> @@ -215,6 +215,24 @@ void __cpuidle acpi_processor_ffh_cstate_enter(struct acpi_processor_cx *cx)
>  }
>  EXPORT_SYMBOL_GPL(acpi_processor_ffh_cstate_enter);
>  
> +static int acpi_processor_ffh_play_dead(struct acpi_processor_cx *cx)
> +{
> +       unsigned int cpu = smp_processor_id();
> +       struct cstate_entry *percpu_entry;
> +
> +       /*
> +        * This is ugly. But AMD processors don't prefer MWAIT based
> +        * C-states when processors are offlined.
> +        */
> +       if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
> +           boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)
> +               return -ENODEV;

Are there AMD systems with FFh idle states at all?

Also, I don't think this works right, the loop in cpuidle_play_dead()
will terminate on this, and not try a shallower state which might have
IO/HLT on.

> +
> +       percpu_entry = per_cpu_ptr(cpu_cstate_entry, cpu);
> +       return mwait_play_dead_with_hints(percpu_entry->states[cx->index].eax);
> +}
> +EXPORT_SYMBOL_GPL(acpi_processor_ffh_play_dead);