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: <CAJZ5v0jeJsGyvb-LU7=sy37Cgj_SUCEGirAF58q5h_5ymx3juA@mail.gmail.com>
Date: Thu, 14 Nov 2024 12:58:49 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>, "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, 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 5:27 PM Wysocki, Rafael J
<rafael.j.wysocki@...el.com> wrote:
>
>
> On 11/13/2024 5:22 PM, Peter Zijlstra wrote:
> > 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?
>
> I don't know.
>
>
> > 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.
>
> I think you are right.

AFAICS, cpuidle_play_dead() needs to be reworked to not bail out after
receiving an error from :play_dead() for one state, but only after all
of them have failed.

> >> +
> >> +       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);