linux-kernel - Re: [PATCH RESEND 0/3] Represent cluster topology and enable load balance between clusters

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Date:   Tue, 5 Oct 2021 10:04:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Barry Song <21cnbao@...il.com>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ben Segall <bsegall@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Guodong Xu <guodong.xu@...aro.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        "Cc: Len Brown" <lenb@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        Linuxarm <linuxarm@...wei.com>,
        Mark Rutland <mark.rutland@....com>,
        Mel Gorman <mgorman@...e.de>, msys.mizuma@...il.com,
        "Zengtao (B)" <prime.zeng@...ilicon.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Sudeep Holla <sudeep.holla@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Will Deacon <will@...nel.org>, x86 <x86@...nel.org>,
        yangyicong <yangyicong@...wei.com>
Subject: Re: [PATCH RESEND 0/3] Represent cluster topology and enable load
 balance between clusters

On Sat, Oct 02, 2021 at 08:09:58PM +1300, Barry Song wrote:

> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 7e4651a1aaf4..86821e83b935 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -993,8 +993,13 @@ config SCHED_CLUSTER
>         bool "Cluster scheduler support"
>         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. For example, on Hisilicon Kunpeng920, each 4 CPUs share
> +         LLC cache tags. This feature isn't a universal win because it
> +         can bring a cost of slightly increased overhead in some places.
> +         If unsure say N here.
> 
>  config SCHED_SMT
>         bool "SMT scheduler support"
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index bd27b1cdac34..940eb1fe0abb 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1002,12 +1002,17 @@ config NR_CPUS
>           to the kernel image.
> 
>  config SCHED_CLUSTER
> +       def_bool y
> +       prompt "Cluster scheduler support"
>         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. For example, on x86 Jacobsville, each 4 CPUs share one
> +        L2 cache. 

			This feature isn't a universal win because it can bring
> +        a cost of slightly increased overhead in some places. If unsure
> +        say N here.

That is a really odd addition to a default-y feature.

How about I make both:

	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.