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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 8/26/25 9:43 AM, K Prateek Nayak wrote:
> PowerPC enables the MC scheduling domain by default on systems with
> coregroup support without having a SCHED_MC config in Kconfig.
> 
> The scheduler uses CONFIG_SCHED_MC to introduce the MC domain in the
> default topology (core) and to optimize the default CPU selection
> routine (sched-ext).

Curious to know if sched_ext usage. i see below code.

if (static_branch_maybe(CONFIG_SCHED_MC, &scx_selcpu_topo_llc))

scx_selcpu_topo_llc = true if there is sd_llc. One can have llc domain without MC domain.
I am wondering whats the reason behind the clubbing.

> 
> Introduce CONFIG_SCHED_MC for powerpc and note that it should be
> preferably enabled given the current default behavior. This also ensures
> PowerPC is tested during future developments that come to depend on
> CONFIG_SCHED_MC.
> 
> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
>   arch/powerpc/Kconfig           | 9 +++++++++
>   arch/powerpc/include/asm/smp.h | 2 ++
>   arch/powerpc/kernel/smp.c      | 4 ++++
>   3 files changed, 15 insertions(+)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 93402a1d9c9f..e954ab3f635f 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -971,6 +971,15 @@ config SCHED_SMT
>   	  when dealing with POWER5 cpus at a cost of slightly increased
>   	  overhead in some places. If unsure say N here.
>   
> +config SCHED_MC
> +	bool "Multi-Core Cache (MC) scheduler support"
> +	depends on PPC64 && SMP
> +	default y
> +	help
> +	  MC scheduler support improves the CPU scheduler's decision making
> +	  when dealing with POWER systems that contain multiple Last Level
> +	  Cache instances on the same socket. If unsure say Y here.
> +
>   config PPC_DENORMALISATION
>   	bool "PowerPC denormalisation exception handling"
>   	depends on PPC_BOOK3S_64
> diff --git a/arch/powerpc/include/asm/smp.h b/arch/powerpc/include/asm/smp.h
> index 86de4d0dd0aa..9a320d96e891 100644
> --- a/arch/powerpc/include/asm/smp.h
> +++ b/arch/powerpc/include/asm/smp.h
> @@ -148,7 +148,9 @@ static inline const struct cpumask *cpu_smt_mask(int cpu)
>   }
>   #endif /* CONFIG_SCHED_SMT */
>   
> +#ifdef CONFIG_SCHED_MC
>   extern const struct cpumask *cpu_coregroup_mask(int cpu);
> +#endif
>   

Is ifdef necessary here?

>   /* Since OpenPIC has only 4 IPIs, we use slightly different message numbers.
>    *
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index e623f2864dc4..7f79b853b221 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1059,6 +1059,7 @@ static bool has_coregroup_support(void)
>   	return coregroup_enabled;
>   }
>   
> +#ifdef CONFIG_SCHED_MC
>   const struct cpumask *cpu_coregroup_mask(int cpu)
>   {
>   	if (has_coregroup_support())
> @@ -1071,6 +1072,7 @@ static const struct cpumask *cpu_mc_mask(struct sched_domain_topology_level *tl,
>   {
>   	return cpu_corgrp_mask(cpu);
>   }
> +#endif
>   

Previous patch says cpu_coregroup_mask is exported. Is it exported in any way to user or modules?

Also i don't see similar gating in other archs. It maybe unnecessary.

>   static const struct cpumask *cpu_pkg_mask(struct sched_domain_topology_level *tl, int cpu)
>   {
> @@ -1729,10 +1731,12 @@ static void __init build_sched_topology(void)
>   			SDTL_INIT(shared_cache_mask, powerpc_shared_cache_flags, CACHE);
>   	}
>   
> +#ifdef CONFIG_SCHED_MC
>   	if (has_coregroup_support()) {
>   		powerpc_topology[i++] =
>   			SDTL_INIT(cpu_mc_mask, powerpc_shared_proc_flags, MC);
>   	}
> +#endif

Just this gating should suffice IMO.>   
>   	powerpc_topology[i++] = SDTL_INIT(cpu_pkg_mask, powerpc_shared_proc_flags, PKG);
>