linux-kernel - Re: [PATCH v4 0/7] Add support for Sub-NUMA cluster (SNC) systems

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Message-ID: <e350514e-76ed-14ea-3e74-c0852658182f@intel.com>
Date:   Fri, 11 Aug 2023 10:27:04 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
        "Peter Newman" <peternewman@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        "Shuah Khan" <skhan@...uxfoundation.org>, <x86@...nel.org>
CC:     Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Babu Moger <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <patches@...ts.linux.dev>
Subject: Re: [PATCH v4 0/7] Add support for Sub-NUMA cluster (SNC) systems

Hi Tony,

On 7/22/2023 12:07 PM, Tony Luck wrote:
> The Sub-NUMA cluster feature on some Intel processors partitions
> the CPUs that share an L3 cache into two or more sets. This plays
> havoc with the Resource Director Technology (RDT) monitoring features.
> Prior to this patch Intel has advised that SNC and RDT are incompatible.
> 
> Some of these CPU support an MSR that can partition the RMID
> counters in the same way. This allows for monitoring features
> to be used (with the caveat that memory accesses between different
> SNC NUMA nodes may still not be counted accuratlely.

accuratlely. -> accurately).

Is there any guidance on the scenarios under which memory accesses
may not be counted accurately, how users can detect when this
is the case, or any techniques users can use to avoid this?

Since this question has come up during this series I do think it
will help to document the impact of SNC on CAT.

Reinette