Re: [PATCH v7 4/8] powerpc/smp: Introduce CONFIG_SCHED_MC to guard MC scheduling bits

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 14, 2025 at 11:25:53AM +0200, Geert Uytterhoeven wrote:
> Hoi Peter,
> 
> On Thu, 28 Aug 2025 at 14:57, Peter Zijlstra <peterz@infradead.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 :/

> > +       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.

> 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?