linux-kernel - RE: [PATCH] ACPI: processor_idle: Skip dummy wait for processors based on the Zen microarchitecture

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Date:   Thu, 22 Sep 2022 18:28:48 +0000
From:   "Limonciello, Mario" <Mario.Limonciello@....com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "Nayak, K Prateek" <KPrateek.Nayak@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "rafael@...nel.org" <rafael@...nel.org>,
        "lenb@...nel.org" <lenb@...nel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "andi@...as.de" <andi@...as.de>, "puwen@...on.cn" <puwen@...on.cn>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rui.zhang@...el.com" <rui.zhang@...el.com>,
        "gpiccoli@...lia.com" <gpiccoli@...lia.com>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "Narayan, Ananth" <Ananth.Narayan@....com>,
        "Shenoy, Gautham Ranjal" <gautham.shenoy@....com>,
        "Ong, Calvin" <Calvin.Ong@....com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "regressions@...ts.linux.dev" <regressions@...ts.linux.dev>
Subject: RE: [PATCH] ACPI: processor_idle: Skip dummy wait for processors
 based on the Zen microarchitecture

[Public]

> -----Original Message-----
> From: Dave Hansen <dave.hansen@...el.com>
> Sent: Thursday, September 22, 2022 13:18
> To: Limonciello, Mario <Mario.Limonciello@....com>; Nayak, K Prateek
> <KPrateek.Nayak@....com>; linux-kernel@...r.kernel.org
> Cc: rafael@...nel.org; lenb@...nel.org; linux-acpi@...r.kernel.org; linux-
> pm@...r.kernel.org; dave.hansen@...ux.intel.com; bp@...en8.de;
> tglx@...utronix.de; andi@...as.de; puwen@...on.cn; peterz@...radead.org;
> rui.zhang@...el.com; gpiccoli@...lia.com; daniel.lezcano@...aro.org;
> Narayan, Ananth <Ananth.Narayan@....com>; Shenoy, Gautham Ranjal
> <gautham.shenoy@....com>; Ong, Calvin <Calvin.Ong@....com>;
> stable@...r.kernel.org; regressions@...ts.linux.dev
> Subject: Re: [PATCH] ACPI: processor_idle: Skip dummy wait for processors
> based on the Zen microarchitecture
> 
> On 9/22/22 10:48, Limonciello, Mario wrote:
> >
> > 2) The title says to limit it to old intel systems, but nothing about this
> actually enforces that.
> > It actually is limited to all Intel systems, but effectively won't be used on
> anything but new
> > ones because of intel_idle.
> >
> > As an idea for #2 you could check for CONFIG_INTEL_IDLE in the Intel case
> and
> > if it's not defined show a pr_notice_once() type of message trying to tell
> people to use
> > Intel Idle instead for better performance.
> 
> What does that have to do with *this* patch, though?

It was just a thought triggered by your commit message title.

> 
> If you've got CONFIG_INTEL_IDLE disabled, you'll be slow before this
> patch.  You'll also be slow after this patch.  It's entirely orthogonal.
> 

Yeah it's orthogonal, but with this discussion happening and the code is
changing /anyway/ then a pr_notice_once() seemed like a nice way to
guide people towards intel_idle at the same time so they didn't trip into
the same problem AMD systems do today.

> I can add a "Practically" to the subject so folks don't confuse it with
> some hard limit that is being enforced:
> 
> 	ACPI: processor idle: Practically limit "Dummy wait" workaround to
> old
> Intel systems

That works.

> 
> BTW, is there seriously a strong technical reason that AMD systems are
> still using this code?  Or is it pure inertia?

Maybe a better question for Ananth and Prateek to comment on.