linux-kernel - Re: [PATCH v1 0/2] x86/smp: Fix power regression introduced by commit 96040f7273e2

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Message-ID: <CAJZ5v0iokGEr6y38P0ic-hqa0A8VVRhxvsUz38m5douwiX+MKA@mail.gmail.com>
Date: Fri, 30 May 2025 19:55:17 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>, x86 Maintainers <x86@...nel.org>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>, LKML <linux-kernel@...r.kernel.org>, 
	Linux PM <linux-pm@...r.kernel.org>, Len Brown <lenb@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...e.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, 
	Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>, 
	"Gautham R. Shenoy" <gautham.shenoy@....com>, Ingo Molnar <mingo@...hat.com>, 
	Todd Brandt <todd.e.brandt@...ux.intel.com>
Subject: Re: [PATCH v1 0/2] x86/smp: Fix power regression introduced by commit 96040f7273e2

On Fri, May 30, 2025 at 6:59 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Fri, May 30, 2025 at 11:18 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Fri, May 30, 2025 at 10:07 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Thu, May 29, 2025 at 11:38:05AM +0200, Rafael J. Wysocki wrote:
> > >
> > > > First off, I'm not sure if all of the requisite things are ready then
> > > > (sysfs etc.).
> > >
> > > Pretty much everything is already running at early_initcall(). Sysfs
> > > certainly is.
> > >
> > > > We may end up doing this eventually, but it may not be straightforward.
> > > >
> > > > More importantly, this is not a change for 6.15.y (y > 0).
> > >
> > > Seriously, have you even tried?
> > >
> > > AFAICT the below is all that is needed.  That boots just fine on the one
> > > randon system I tried, and seems to still work.
> > >
> > > And this is plenty small enough to go into 6.15.y
> >
> > But there is still intel_idle_init() which is device_initcall() ATM
> > and this needs to be tested on other arches too.
>
> intel_idle_init() depends on ACPI which initializes via a
> subsys_initcall() and doing that earlier would mean a major redesign.
>
> Pretty much same for reordering APs initialization past ACPI initialization.
>
> One more option is to kick the "dead" SMT siblings after the idle
> driver initializes and let them do "play dead" again, but who'd be
> responsible for doing that?

Interestingly enough, a similar issue is there during resume from
hibernation and that's why there is arch_resume_nosmt().

So arguably intel_idle_init() could do something analogous to it after
successfully registering the idle driver.  In principle, the ACPI idle
driver could do that too on x86.

Opinions?