Re: [RFC] mpam,x86,fs/resctrl: Generic schema description Proof of Concept

[Reinette Chatre]

Hi Shaopeng Tan,

On 6/18/26 6:42 PM, Shaopeng Tan (Fujitsu) wrote:
> 
>> On 6/11/26 6:30 PM, Shaopeng Tan (Fujitsu) wrote:

>>> Second, regarding the naming of "mbm_total_bytes",
>>> the meaning of this name seems to differ from what is typically found under mon_data/mon_L3_<id>/mbm_total_bytes.
>>
>> Could you please elaborate how the meaning is different? The only information I have about
>> the systems needing this is in Nvidia's portion of
>> https://lpc.events/event/19/contributions/2093/attachments/1958/4172/resctrl%20Microconference%20LPC%202025%20Tokyo.pdf
>>
>>> To avoid confusion and better reflect its different nature,
>>> how about considering an alternative name such as mbm_global_bytes or another more appropriate identifier.
> 
> Current MBM events in resctrl are per L3 to monitor MB between L3 and memory,
> and mbm_total_bytes monitors the L3 total external bandwidth to the next level of the memory hierarchy.
> Is my understanding of this correct?

It is my understanding also.

> 
> Based on NVIDIA's information, they even assume that CPU-less nodes will not feature an L3 cache. 
> I believe NVIDIA's objective is to collect traffic data for each NODE/NUMA, 
> without needing to distinguish which specific L3 cache the traffic originated from.

That is what associating monitoring with scope (instead of L3 resource), and supporting node scope, aims
to address.

> 
> This aligns with the fact that, in ARM architectures (e.g., on NVIDIA's platforms or Fujitsu's MONAKA),
> the MPAM MSC (Memory System Component) can also be located on the memory controller.
> In such cases,it becomes impossible to distinguish which L3 cache the traffic came from.

This is not clear to me. Is the goal to expose the bandwidth at each memory controller?

> 
> Therefore, the proposed 'mbm_global_bytes' refers to account for traffic from all L3 cache within the global system.
> 

As I see it the directory name would make the monitoring scope clear, whether it is
at L3 scope or NUMA node scope. Within the directory there would be the individual files
that should be interpreted based on the scope of the directory they are in. I thus
see a "mbm_total_bytes" in a "L3 scope" directory to have different meaning
from a "mbm_total_bytes" in a "NUMA node scope" directory. 

Reinette