Re: [PATCH v4 5/5] arm64: mpam: Add memory bandwidth usage (MBWU) documentation

[Fenghua Yu <fenghuay@nvidia.com>]

Hi, Ben,

On 7/2/26 07:58, Ben Horgan wrote:
> Hi Reinette,
> 
> On 7/2/26 15:46, Reinette Chatre wrote:
>> Hi Ben,
>>
>> On 7/2/26 2:20 AM, Ben Horgan wrote:
>>> On 7/1/26 23:38, Reinette Chatre wrote:
>>>> On 5/20/26 2:24 PM, Ben Horgan wrote:
>>
>> ...
>>
>>>>> --- a/Documentation/arch/arm64/mpam.rst
>>>>> +++ b/Documentation/arch/arm64/mpam.rst
>>>>> @@ -65,6 +65,23 @@ The supported features are:
>>>>>     there is at least one CSU monitor on each MSC that makes up the L3 group.
>>>>>     Exposing CSU counters from other caches or devices is not supported.
>>>>>   
>>>>> +* Memory Bandwidth Usage (MBWU) on or after the L3 cache.  resctrl uses the
>>>>> +  L3 cache-id to identify where the memory bandwidth is measured. For this
>>>>> +  reason the platform must have an L3 cache with cache-id's supplied by
>>>>> +  firmware. (It doesn't need to support MPAM.)

s/It/The platform/?

>>>>> +
>>>>> +  Memory bandwidth monitoring makes use of MBWU monitors in each MSC that
>>>>> +  makes up the L3 group. If the memory bandwidth monitoring is on the memory
>>>>> +  rather than the L3 then there must be a single global L3 as otherwise it

s/a single global L3/a single global L3 cache id/?

>>>>> +  is unknown which L3 the traffic came from.
>>>>> +
>>>>> +  To expose 'mbm_total_bytes', the topology of the group of MSC chosen must
>>>>> +  match the topology of the L3 cache so that the cache-id's can be
>>>>> +  repainted. For example: Platforms with Memory bandwidth monitors on
>>>>> +  CPU-less NUMA nodes cannot expose 'mbm_total_bytes' as these nodes do not
>>>>> +  have a corresponding L3 cache. 'mbm_local_bytes' is not exposed as MPAM

Maybe remove the CPU-less example here since you will add CPU-less info 
later?

The CPU-less patches will update this document accordingly.

>>>>> +  cannot distinguish local traffic from global traffic.
>>>>
>>>> Hopefully we can get to a point where memory bandwidth monitoring data from
>>>> CPU-less NUMA nodes can be exposed via resctrl. When considering such possible
>>>
>>> Thank you for your interest here. I hope so too.
>>>
>>>> future I think it may make this work easier to build on if the documentation
>>>> focuses on what the current implementation supports and leave room for
>>>> future enhancements by not constraining user space expectation with an absolute
>>>> like "CPU-less NUMA nodes cannot expose 'mbm_total_bytes'".
>>>
>>> The intention was to describe the current limitations but I do see how
>>> this can come across as fundamental problems rather than just that we
>>> need to do some more work to establish how this can be done and
>>> implement it.
>>>
>>> How about if I add this paragraph at the end?
>>>
>>> All these restrictions based on L3 cache are due to resctrl, currently,
>>> only supporting monitoring at the scope of the L3 scope. It is expected
>>
>> How about "at L3 scope" instead of "at the scope of the L3 scope"?
> 
> Sure, that reads better.
> 
> Ben
> 
>>
>>> that going forward more MBWU monitors can be exposed to the user after
>>> support for more monitoring scopes is added to resctrl.
>> Looks good to me, thank you.
>>
>> Reinette
> 
Thanks.

-Fenghua