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

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Message-ID: <20250901085236.GK4068168@noisy.programming.kicks-ass.net>
Date: Mon, 1 Sep 2025 10:52:36 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
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Subject: Re: [PATCH v7 4/8] powerpc/smp: Introduce CONFIG_SCHED_MC to guard
 MC scheduling bits

On Mon, Sep 01, 2025 at 10:35:07AM +0200, Peter Zijlstra wrote:
> On Thu, Aug 28, 2025 at 08:13:51PM +0530, Shrikanth Hegde wrote:
> 
> > > --- a/arch/powerpc/Kconfig
> > > +++ b/arch/powerpc/Kconfig
> > > @@ -170,6 +170,9 @@ config PPC
> > >   	select ARCH_STACKWALK
> > >   	select ARCH_SUPPORTS_ATOMIC_RMW
> > >   	select ARCH_SUPPORTS_DEBUG_PAGEALLOC	if PPC_BOOK3S || PPC_8xx
> > > +	select ARCH_SUPPORTS_SCHED_SMT		if PPC64 && SMP
> > > +	select ARCH_SUPPORTS_SCHED_MC		if PPC64 && SMP
> > > +	select SCHED_MC				if ARCH_SUPPORTS_SCHED_MC
> > 
> > Wondering if this SCHED_MC is necessary here? shouldn't it be set by arch/Kconfig?
> 
> Ah, so without this SCHED_MC becomes a user selectable option, with this
> it is an always on option (for ppc64) -- no user prompt.
> 
> That is, this is the only way I found to have similar semantics to this:
> 
> > > -config SCHED_MC
> > > -	def_bool y
> > > -	depends on PPC64 && SMP
> > > -
> 
> Which is also not a user selectable option.
> 
> > nit: Also, can we have so they are still sorted?
> > 	select ARCH_SUPPORTS_SCHED_MC		if PPC64 && SMP
> > 	select ARCH_SUPPORTS_SCHED_SMT		if PPC64 && SMP
> 
> Sure, let me flip them. I need to prod that that patch anyway, built
> robot still ain'ted happy.

Looks like 44x/iss476-smp_defconfig (iow 32bit power) also wants
SCHED_MC, so it should be:

config SCHED_MC
	def_bool y
	depends on SMP

Its just SMT that's a PPC64 special.