Re: [PATCH] ACPI: processor_idle: Skip dummy wait for processors based on the Zen microarchitecture

[Andreas Mohr]

Hi,

On Thu, Sep 22, 2022 at 10:01:46AM -0700, Dave Hansen wrote:
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index 16a1663d02d4..9f40917c49ef 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -531,10 +531,27 @@ static void wait_for_freeze(void)
>  	/* No delay is needed if we are in guest */
>  	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
>  		return;
> +	/*
> +	 * Modern (>=Nehalem) Intel systems use ACPI via intel_idle,
> +	 * not this code.  Assume that any Intel systems using this
> +	 * are ancient and may need the dummy wait.  This also assumes
> +	 * that the motivating chipset issue was Intel-only.
> +	 */
> +	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
> +		return;
>  #endif
> -	/* Dummy wait op - must do something useless after P_LVL2 read
> -	   because chipsets cannot guarantee that STPCLK# signal
> -	   gets asserted in time to freeze execution properly. */

16 years ago,
I did my testing on a VIA 8233/8235 chipset (AMD Athlon/Duron) system......
(plus reading VIA spec PDFs which mentioned "STPCLK#" etc.).




AFAIR I was doing kernel profiling (via oprofile, IIRC)
for painful performance hotspots (read: I/O accesses etc.), and
this was one resulting place which I stumbled over.
And if I'm not completely mistaken,
that dummy wait I/O op *was* needed (else "nice" effects)
on my system (put loud and clear: *non*-Intel).



So one can see where my profiling effort went
(*optimizing* things, not degrading them)
--> hints that current Zen3-originating effort is not
about a regression in the "regression bug" sense -
merely a (albeit rather appreciable/sizeable... congrats!)
performance deterioration vs.
an optimal (currently non-achieved) software implementation state
(also: of PORT-based handling [vs. MWAIT], mind you!).


I still have that VIA hardware, but inactive
(had the oh-so-usual capacitors issue :( ).


Sorry for sabotaging your current fix efforts ;-) -
but thank you very much for your work/discussion
in this very central/hotpath area! (this extends to all of you...)

Greetings

Andreas Mohr