linux-kernel - Re: [PATCH v6 0/8] Add support for Sub-NUMA cluster (SNC) systems

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Date:   Fri, 29 Sep 2023 14:06:19 -0700
From:   Tony Luck <tony.luck@...el.com>
To:     Peter Newman <peternewman@...gle.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>, x86@...nel.org,
        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 v6 0/8] Add support for Sub-NUMA cluster (SNC) systems

On Fri, Sep 29, 2023 at 04:33:17PM +0200, Peter Newman wrote:
> Hi Tony,
> 
> On Thu, Sep 28, 2023 at 9:14 PM Tony Luck <tony.luck@...el.com> 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.
> 
> Is an "SNC NUMA node" a "sub-NUMA node", or a NUMA node on which SNC
> has been enabled?

It would be architecturally possible to enable SNC mode on
a subset of CPU sockets. But there isn't a BIOS setup option
to do that. You either have SNC everywhere, or nowhere.

I prefer "SNC NUMA node" == "sub-NUMA node".

This version "NUMA node on which SNC has been enabled"
makes it sound like there is a control on a NUMA node
that can be switched.  The control is on the CPU socket.
That's often equivalent to a NUMA node, but Intel has
had CPUs in the past where this isn't the case (e.g.
Cascade Lake -AP and Cooper Lake).
> 
> Thanks!
> -Peter


Thanks for the review of the series. I've applied changes
to my local tree. Will post v7 of the series early next
week if no other reviews come in.

-Tony