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Date:   Fri, 15 Oct 2021 15:04:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Barry Song <21cnbao@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        linux-ia64@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
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        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86 <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
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        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
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        Arnd Bergmann <arnd@...db.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
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        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        YiFei Zhu <yifeifz2@...inois.edu>,
        Uwe Kleine-K├Ânig 
        <u.kleine-koenig@...gutronix.de>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Sergei Trofimovich <slyfox@...too.org>,
        David Hildenbrand <david@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
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        Vlastimil Babka <vbabka@...e.cz>,
        Frederic Weisbecker <frederic@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH 2/2] sched: Centralize SCHED_{SMT, MC, CLUSTER}
 definitions

On Fri, Oct 08, 2021 at 04:22:27PM +0100, Valentin Schneider wrote:

> So x86 has it default yes, and a lot of others (e.g. arm64) have it default
> no.
> 
> IMO you don't gain much by disabling them. SCHED_MC and SCHED_CLUSTER only
> control the presence of a sched_domain_topology_level - if it's useless it
> gets degenerated at domain build time. Some valid reasons for not using
> them is if the architecture defines its own topology table (e.g. powerpc
> has CACHE and MC levels which are not gated behind any CONFIG).
> 
> SCHED_SMT has an impact on code generated in sched/core.c, but that is also
> gated by a static key.
> 
> So I'd say having them default yes is sensible. I'd even say we should
> change the "If unsure say N here." to "Y".

Right, so I tend to agree (and also that we should fix that Kconfig help
text). But it would be very nice to have feedback from the affected arch
maintainers.

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