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

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Message-ID: <CAMuHMdWMkKFLEZ34j=JV0Ls+o80UfC6s3yD5x-9G2K8ZV-Y-9w@mail.gmail.com>
Date: Tue, 14 Oct 2025 11:25:53 +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>, 
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	"H. Peter Anvin" <hpa@...or.com>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, linuxppc-dev@...ts.ozlabs.org, 
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	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 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...

> +       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?
If it is really needed on some architectures or platforms, I guess
they can still select it explicitly?

> +
> +config SCHED_CLUSTER
> +       bool "Cluster scheduler support"
> +       depends on ARCH_SUPPORTS_SCHED_CLUSTER
> +       default y

Likewise.

> +       help
> +         Cluster scheduler support improves the CPU scheduler's decision
> +         making when dealing with machines that have clusters of CPUs.
> +         Cluster usually means a couple of CPUs which are placed closely
> +         by sharing mid-level caches, last-level cache tags or internal
> +         busses.
> +
> +config SCHED_MC
> +       bool "Multi-Core Cache (MC) scheduler support"
> +       depends on ARCH_SUPPORTS_SCHED_MC
> +       default y

Likewise.

> +       help
> +         Multi-core scheduler support improves the CPU scheduler's decision
> +         making when dealing with multi-core CPU chips at a cost of slightly
> +         increased overhead in some places. If unsure say N here.

Likewise.

> +
>  # Selected by HOTPLUG_CORE_SYNC_DEAD or HOTPLUG_CORE_SYNC_FULL
>  config HOTPLUG_CORE_SYNC
>         bool

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