linux-kernel - Re: [PATCH v7 4/8] powerpc/smp: Introduce CONFIG_SCHED

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Message-ID: <CAMuHMdUD8RZqPL5ZYyJrwJB+XL_Tkn-rsLx7WvUmn6y5M_tAtw@mail.gmail.com>
Date: Tue, 14 Oct 2025 14:37:11 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>, K Prateek Nayak <kprateek.nayak@....com>, 
	Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, 
	Nicholas Piggin <npiggin@...il.com>, Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Alexander Gordeev <agordeev@...ux.ibm.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Sven Schnelle <svens@...ux.ibm.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, linuxppc-dev@...ts.ozlabs.org, 
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
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	Valentin Schneider <vschneid@...hat.com>, thomas.weissschuh@...utronix.de, 
	Li Chen <chenl311@...natelecom.cn>, Bibo Mao <maobibo@...ngson.cn>, 
	Mete Durlu <meted@...ux.ibm.com>, Tobias Huschle <huschle@...ux.ibm.com>, 
	Easwar Hariharan <easwar.hariharan@...ux.microsoft.com>, 
	Guo Weikang <guoweikang.kernel@...il.com>, 
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>, Brian Gerst <brgerst@...il.com>, 
	Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>, Swapnil Sapkal <swapnil.sapkal@....com>, 
	"Yury Norov [NVIDIA]" <yury.norov@...il.com>, Sudeep Holla <sudeep.holla@....com>, 
	Jonathan Cameron <Jonathan.Cameron@...wei.com>, Andrea Righi <arighi@...dia.com>, 
	Yicong Yang <yangyicong@...ilicon.com>, 
	Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>, 
	Tim Chen <tim.c.chen@...ux.intel.com>, 
	Vinicius Costa Gomes <vinicius.gomes@...el.com>
Subject: Re: [PATCH v7 4/8] powerpc/smp: Introduce CONFIG_SCHED_MC to guard MC
 scheduling bits

Hoi Peter,

On Tue, 14 Oct 2025 at 11:42, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Oct 14, 2025 at 11:25:53AM +0200, Geert Uytterhoeven wrote:
> > On Thu, 28 Aug 2025 at 14:57, Peter Zijlstra <peterz@...radead.org> wrote:
> > > Now, when I look at unifying those config options (there's a metric ton
> > > of crap that's duplicated in the arch/*/Kconfig), I end up with something
> > > like the below.
> > >
> > > And while that isn't exact, it is the closest I could make it without
> > > making a giant mess of things.
> > >
> > > WDYT?
> >
> > Thanks for your patch, which is now commit 7bd291abe2da09f5 ("sched:
> > Unify the SCHED_{SMT,CLUSTER,MC} Kconfig") in v6.18-rc1.
> >
> > > --- a/arch/Kconfig
> > > +++ b/arch/Kconfig
> > > @@ -41,6 +41,44 @@ config HOTPLUG_SMT
> > >  config SMT_NUM_THREADS_DYNAMIC
> > >         bool
> > >
> > > +config ARCH_SUPPORTS_SCHED_SMT
> > > +       bool
> > > +
> > > +config ARCH_SUPPORTS_SCHED_CLUSTER
> > > +       bool
> > > +
> > > +config ARCH_SUPPORTS_SCHED_MC
> > > +       bool
> > > +
> > > +config SCHED_SMT
> > > +       bool "SMT (Hyperthreading) scheduler support"
> > > +       depends on ARCH_SUPPORTS_SCHED_SMT
> > > +       default y
> >
> > This is now enabled by default everywhere, while it was disabled by
> > default on most architectures before...
>
> I'm not sure ARCH_SUPPORTS_SCHED_SMT counts as everywhere, but yes.
> A fair deal of the architectures had all this default yes, and I had to
> pick something. Can't make an omelette without breaking an egg and all
> that :/

OK.

> > > +       help
> > > +         Improves the CPU scheduler's decision making when dealing with
> > > +         MultiThreading at a cost of slightly increased overhead in some
> > > +         places. If unsure say N here.
> >
> > So it should default to n?
>
> That's just help text that got carried around. Many of the architectures
> that had default y still had this text on. I suppose we can change it if
> someone cares.

Please do so.

> > If it is really needed on some architectures or platforms, I guess
> > they can still select it explicitly?
>
> There were 4 cases:
>
>  - arch doesn't support SMT
>  - arch supports SMT and lets user pick, default Y
>  - arch supports SMT and lets user pick, default N
>  - arch mandates SMT
>
> Of those 3 are still possible, the one we lost is the default N case.
>
> Old configs that have =N will continue to have N. New configs might end
> up with Y.
>
> Why is this a problem?

While old .config files will see no changes, old defconfig files do
need updates.

Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds