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

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Message-ID: <0b54618f-ce15-575a-9571-f746e8bc40e2@amd.com>
Date:   Tue, 11 Apr 2023 16:25:23 +0530
From:   K Prateek Nayak <kprateek.nayak@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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

Hello Peter,

Thank you for reviewing the patches.

On 4/11/2023 1:55 PM, Peter Zijlstra wrote:
> 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?)

I can reword the cluster description as follows:

"A cluster consists of threads of one or more cores sharing the same
L2 cache."

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

I hope the above rewording solves this confusion. Let me know otherwise.
--
Thanks and Regards,
Prateek