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

[Reinette Chatre]

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.)
>>> +
>>> +  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
>>> +  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
>>> +  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"?

> 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