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

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Message-ID: <20250813-gifted-nimble-wildcat-6cdf65@sudeepholla>
Date: Wed, 13 Aug 2025 10:28:47 +0100
From: Sudeep Holla <sudeep.holla@....com>
To: Jeremy Linton <jeremy.linton@....com>
Cc: "Christoph Lameter (Ampere)" <cl@...two.org>,
	Sudeep Holla <sudeep.holla@....com>,
	Huang Shijie <shijie@...amperecomputing.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 Tue, Aug 12, 2025 at 12:32:36PM -0500, Jeremy Linton wrote:
> On 8/12/25 11:33 AM, Christoph Lameter (Ampere) wrote:
> > On Mon, 11 Aug 2025, Jeremy Linton wrote:
> > 
> > >  From what I've seen, SCHED_CLUSTER seems to be a bit of give and take
> > > depending on benchmark and machine. I'm not sure if it should be default
> > > enabled or not, but it would really be nice to have at least a larger sweep of
> > > benchmarks/machines in order to be sure of the decision.
> > 
> > If the hardware provides a clusterid then I think this clusterid should be
> > used for the sched domains. CONFIG_SCHED_CLUSTER does that. So it should
> > be the default.
> 
> Hi,
> 
> 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.

-- 
Regards,
Sudeep