Re: [PATCH v3 2/3] x86/smp native_play_dead: Prefer cpuidle_play_dead() over mwait_play_dead()

From: Rafael J. Wysocki
Date: Thu Nov 14 2024 - 06:59:11 EST


On Wed, Nov 13, 2024 at 5:27 PM Wysocki, Rafael J
<rafael.j.wysocki@xxxxxxxxx> 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);