linux-kernel - Re: [PATCH v2 2/8] arch_topology: Set thread sibling cpumask only within the cluster

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Message-ID: <db6dc5e0-780a-5542-5e49-711cd454a9d8@arm.com>
Date:   Fri, 20 May 2022 14:32:19 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Sudeep Holla <sudeep.holla@....com>,
        Atish Patra <atishp@...shpatra.org>,
        linux-kernel@...r.kernel.org
Cc:     Atish Patra <atishp@...osinc.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qing Wang <wangqing@...o.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v2 2/8] arch_topology: Set thread sibling cpumask only
 within the cluster

On 18/05/2022 11:33, Sudeep Holla wrote:
> Currently the cluster identifier is not set on the DT based platforms.
> The reset or default value is -1 for all the CPUs. Once we assign the
> cluster identifier values correctly that imay result in getting the thread
> siblings wrongs as the core identifiers can be same for 2 different CPUs
> belonging to 2 different cluster.
> 
> So, in order to get the thread sibling cpumasks correct, we need to
> update them only if the cores they belong are in the same cluster within
> the socket. Let us skip updation of the thread sibling cpumaks if the
> cluster identifier doesn't match.

So this issue should be there on ACPI systems as well then. Kunpeng920
and Ampere Altra don't have SMT, so very likely nobody has noticed this
so far.

[...]