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

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Message-ID: <9d3d3424-a6d4-4076-87ff-a1c216de79c6@intel.com>
Date:   Thu, 22 Sep 2022 11:17:53 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Limonciello, Mario" <Mario.Limonciello@....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

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?

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.

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

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