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

[Ben Horgan <ben.horgan@arm.com>]

Hi Reinette,

On 7/1/26 23:38, Reinette Chatre wrote:
> Hi Ben,
> 
> On 5/20/26 2:24 PM, Ben Horgan wrote:
>> Memory bandwidth monitoring make uses of MBWU monitors and is now exposed
>> to the user via resctrl. Add some documentation so the user knows what to
>> expect.
>>
>> Co-developed-by: James Morse <james.morse@arm.com>
>> Signed-off-by: James Morse <james.morse@arm.com>
>> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
>> ---
>>  Documentation/arch/arm64/mpam.rst | 17 +++++++++++++++++
>>  1 file changed, 17 insertions(+)
>>
>> diff --git a/Documentation/arch/arm64/mpam.rst b/Documentation/arch/arm64/mpam.rst
>> index 570f51a8d4eb..208ff17068c4 100644
>> --- 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
that going forward more MBWU monitors can be exposed to the user after
support for more monitoring scopes is added to resctrl.

Thanks,

Ben>
> Reinette