linux-kernel - Re: [PATCH 2/2] x86/Documentation: Add documentation about cluster

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Date:   Tue, 11 Apr 2023 10:25:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     K Prateek Nayak <kprateek.nayak@....com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        corbet@....net, jgross@...e.com, andrew.cooper3@...rix.com,
        Jason@...c4.com, thomas.lendacky@....com, puwen@...on.cn,
        x86@...nel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/Documentation: Add documentation about cluster

On Mon, Apr 10, 2023 at 10:05:27PM +0530, K Prateek Nayak wrote:
> x86 processors map cluster to the L2 cache. Add documentation stating
> the same, and provide more information on the values and API related to
> CPU clusters exposed by the kernel.
> 
> Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
> ---
>  Documentation/x86/topology.rst | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/Documentation/x86/topology.rst b/Documentation/x86/topology.rst
> index 7f58010ea86a..35991d8cdef1 100644
> --- a/Documentation/x86/topology.rst
> +++ b/Documentation/x86/topology.rst
> @@ -33,6 +33,7 @@ historical nature and should be cleaned up.
>  The topology of a system is described in the units of:
>  
>      - packages
> +    - cluster
>      - cores
>      - threads
>  
> @@ -90,6 +91,27 @@ Package-related topology information in the kernel:
>          Cache. In general, it is a number identifying an LLC uniquely on the
>          system.
>  
> +
> +Clusters
> +========
> +A cluster consists of 1 or more threads. It does not matter whether the threads
> +are SMT- or CMT-type threads. All the threads of a cluster share the same L2
> +cache.

I'm not quite sure that's a correct discription of what a cluster is.

Yes, SMT will fundamentally share core-level caches (and should we not
always have SMT share all cache topoligies?)

But there is also x86 where L2 is shared between multiple cores -- while
the above seems to suggest L2 is single core only.