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

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Message-ID: <CAJZ5v0jzF19rToJMHhEvU6Zbt3690KWCs-B_0sPR=s9xeRiUnQ@mail.gmail.com>
Date: Wed, 28 May 2025 19:09:21 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, "Rafael J. Wysocki" <rjw@...ysocki.net>, 
	x86 Maintainers <x86@...nel.org>, 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 Wed, May 28, 2025 at 6:05 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, May 28, 2025 at 04:25:19PM +0200, Rafael J. Wysocki wrote:
>
> > If cpuidle is available and works, it will do the same thing.
>
> Why can't we make it available sooner? But no, cpuidle does not do the
> same thing -- it was argued it does the right thing because it has them
> tables with C states on and doesn't try and divinate from CPUID.
>
> > > The whole point was that mwait_play_dead did not DTRT because hints are
> > > stupid and it could not select the deepest C state in an unambiguous
> > > fashion.
> >
> > Yes, on some systems.
>
> The 'on some systems' thing is irrelevant. Either it always works, or it
> doesn't and we shouldnt be having it.
>
> > > And now you're restoring that -- code you all argued was fundamentally
> > > buggered.
> > >
> > > Yes is 'fixes' things on old platforms, but it is equally broken on the
> > > new platforms where you all argued it was broken on. So either way
> > > around you're going to need to fix those, and this isn't it.
>
> > The commit reverted by the first patch removed
> > mwait_play_dead_cpuid_hint() altogether, so it never runs and the only
> > fallback is hlt_play_dead(), but this doesn't work for disabling SMT
> > siblings.
>
> It should either be fixed to always work or stay dead.

I'm talking about the current code which is broken on many systems.

> > > Now, SMT siblings are all AP, by definition. So can't we simply send
> > > them INIT instead of doing CLI;HLT, that way they drop into
> > > Wait-for-SIPI and the ucode can sort it out?
> >
> > No, I don't think so.  I don't think that Wait-for-SIPI is an idle state.
> >
> > But we are discussing patch [2/2] here while really the problem is
> > that the commit in question is broken, so it needs to be reverted in
> > the first place.
>
> No, you all very much argued that mwait_play_dead couldn't be fixed, as
> such it must die and stay dead. Sometimes working is worse than never
> working.
>
> So no, I very much object to the revert.

And I object to leaving a user-visible regression behind.