linux-kernel - Re: [RFC PATCH] topology: Represent clusters of CPUs within a die.

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Date:   Mon, 19 Oct 2020 12:35:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     linux-acpi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Len Brown <len.brown@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sudeep Holla <sudeep.holla@....com>, guohanjun@...wei.com,
        Will Deacon <will@...nel.org>, linuxarm@...wei.com,
        Brice Goglin <Brice.Goglin@...ia.fr>,
        valentin.schneider@....com
Subject: Re: [RFC PATCH] topology: Represent clusters of CPUs within a die.

On Fri, Oct 16, 2020 at 11:27:02PM +0800, Jonathan Cameron wrote:
> Both ACPI and DT provide the ability to describe additional layers of
> topology between that of individual cores and higher level constructs
> such as the level at which the last level cache is shared.
> In ACPI this can be represented in PPTT as a Processor Hierarchy
> Node Structure [1] that is the parent of the CPU cores and in turn
> has a parent Processor Hierarchy Nodes Structure representing
> a higher level of topology.
> 
> For example Kunpeng 920 has clusters of 4 CPUs.  These do not share
> any cache resources, but the interconnect topology is such that
> the cost to transfer ownership of a cacheline between CPUs within
> a cluster is lower than between CPUs in different clusters on the same
> die.   Hence, it can make sense to deliberately schedule threads
> sharing data to a single cluster.
> 
> This patch simply exposes this information to userspace libraries
> like hwloc by providing cluster_cpus and related sysfs attributes.
> PoC of HWLOC support at [2].
> 
> Note this patch only handle the ACPI case.
> 
> Special consideration is needed for SMT processors, where it is
> necessary to move 2 levels up the hierarchy from the leaf nodes
> (thus skipping the processor core level).

I'm confused by all of this. The core level is exactly what you seem to
want.