linux-kernel - Re: [PATCH 2/2] sched: Centralize SCHED

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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>,
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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.