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

From: Andreas Mohr <andi@lisas.de>
To: Dave Hansen <dave.hansen@intel.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>,
	linux-kernel@vger.kernel.org, rafael@kernel.org, lenb@kernel.org,
	linux-acpi@vger.kernel.org, linux-pm@vger.kernel.org,
	dave.hansen@linux.intel.com, bp@alien8.de, tglx@linutronix.de,
	andi@lisas.de, puwen@hygon.cn, mario.limonciello@amd.com,
	peterz@infradead.org, rui.zhang@intel.com, gpiccoli@igalia.com,
	daniel.lezcano@linaro.org, ananth.narayan@amd.com,
	gautham.shenoy@amd.com, Calvin Ong <calvin.ong@amd.com>,
	stable@vger.kernel.org, regressions@lists.linux.dev
Subject: Re: [PATCH] ACPI: processor_idle: Skip dummy wait for processors based on the Zen microarchitecture
Date: Thu, 22 Sep 2022 21:42:15 +0200	[thread overview]
Message-ID: <Yyy6l94G0O2B7Yh1@rhlx01.hs-esslingen.de> (raw)
In-Reply-To: <88c17568-8694-940a-0f1f-9d345e8dcbdb@intel.com>

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