linux-kernel - Re: [PATCH] arm64: defconfig: enable CONFIG_SCHED

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Message-ID: <aJ21eZPPmQVR3Nij@bogus>
Date: Thu, 14 Aug 2025 11:07:53 +0100
From: Sudeep Holla <sudeep.holla@....com>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Jeremy Linton <jeremy.linton@....com>,
	Huang Shijie <shijie@...amperecomputing.com>,
	Sudeep Holla <sudeep.holla@....com>, catalin.marinas@....com,
	will@...nel.org, patches@...erecomputing.com,
	Shubhang@...amperecomputing.com, krzysztof.kozlowski@...aro.org,
	bjorn.andersson@....qualcomm.com, geert+renesas@...der.be,
	arnd@...db.de, nm@...com, ebiggers@...nel.org,
	nfraprado@...labora.com, prabhakar.mahadev-lad.rj@...renesas.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: defconfig: enable CONFIG_SCHED_CLUSTER

On Wed, Aug 13, 2025 at 08:55:36AM -0700, Christoph Lameter (Ampere) wrote:
> On Wed, 13 Aug 2025, Sudeep Holla wrote:
> 
> > > The problem is that this information is being sourced from the ACPI PPTT.
> > > The ACPI specification (AFAIK) doesn't define a cluster, so the linux
> > > cluster information is being 'invented' based on however the firmware vendor
> > > choose to group CPU nodes in the PPTT. Which means its possible for them to
> > > unknowingly create clusters, or also fail to create them when they make
> > > sense.
> >
> > +1, completely agree. As Jeremy mentioned, it is hit or miss and cluster
> > is loosely defined and IIRC Huawei pushed this based on their platform at
> > the time and it did break some benchmarks on few other platforms. So it
> > is not a good idea to make it default config IMO.
> 
> Can we figure out which platforms benchmarks were affected and why?
> 

I am not sure on either. One way to figure out the affected platforms is
to merge this change and expect the platform users/maintainers to report.

> It seems the notion of a "cluster" on ARM64 is derived (I guess a better
> word than "invented" hehe)  from sibling information instead of PPTT. But
> using that information should work fine right?
> 

I have my doubts but I may be wrong. As mentioned in the other email in this
thread, "cluster" IMO is ill-defined both in ACPI and Arm architecture which
is the root cause for all the issue around it.

-- 
Regards,
Sudeep