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 17:21:21 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>, andi@...as.de,
        Pu Wen <puwen@...on.cn>,
        Mario Limonciello <mario.limonciello@....com>,
        "Zhang, Rui" <rui.zhang@...el.com>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        ananth.narayan@....com, gautham.shenoy@....com,
        Calvin Ong <calvin.ong@....com>,
        Stable <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 Thu, Sep 22, 2022 at 10:17 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Sep 21, 2022 at 09:48:13PM +0200, Borislav Petkov wrote:
> > On Wed, Sep 21, 2022 at 05:00:35PM +0200, Peter Zijlstra wrote:
> > > On Wed, Sep 21, 2022 at 12:06:38PM +0530, K Prateek Nayak wrote:
> > > > Processors based on the Zen microarchitecture support IOPORT based deeper
> > > > C-states.
> > >
> > > I've just gotta ask; why the heck are you using IO port based idle
> > > states in 2022 ?!?! You have have MWAIT, right?
> >
> > They have both. And both is Intel technology. And as I'm sure you
> > know AMD can't do their own thing - they kinda have to follow Intel.
> > Unfortunately.
> >
> > Are you saying modern Intel chipsets don't do IO-based C-states anymore?
>
> I've no idea what they do, but Linux exclusively uses MWAIT on Intel as
> per intel_idle.c.

Well, it can be forced to use ACPI idle instead.

> MWAIT also cuts down on IPIs because it wakes from the TIF write.

Right.