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: <20241114121743.GR6497@noisy.programming.kicks-ass.net>
Date: Thu, 14 Nov 2024 13:17:43 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: "Gautham R. Shenoy" <gautham.shenoy@....com>,
	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 Thu, Nov 14, 2024 at 12:58:49PM +0100, Rafael J. Wysocki wrote:
> 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.

That and ideally we remove that whole ACPI_STATE_C[123] filter on
setting enter_dead. I don't see how that makes sense.