Re: [RFC PATCH v2 00/19] resctrl: Generalize monitor events across resources

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Jinjie,

On 10/29/26 5:35 AM, Jinjie Ruan wrote:
> Resctrl only knows about monitoring at the L3 scope. MPAM monitors are
> attached to a resource in an MSC, and firmware says which cache level that
> resource is. The driver already finds L2 monitors, but resctrl has
> nowhere to show them.
> 
> This is not MPAM specific. RISC-V can also place monitors at several
> points in the system.
> 

One key to this is in the first sentence of this cover letter: "Resctrl only knows
about monitoring at the L3 *scope*.".

When it comes to exposing monitoring data to user space resctrl has been using "L3"
interchangeable as resource and scope. 

The existing resctrl user interface already establishes that the "L3" in
"L3_MON" and the per monitor group "mon_data/mon_L3_xx" refers to the L3 *scope*,
not the L3 resource. More specifically, consider the existing user interface:

# find mon_data/
mon_data/
mon_data/mon_L3_00
mon_data/mon_L3_00/mbm_local_bytes <=== monitoring data associated with MB resource at L3 scope
mon_data/mon_L3_00/mbm_total_bytes <=== monitoring data associated with MB resource at L3 scope
mon_data/mon_L3_00/llc_occupancy   <=== monitoring data associated with L3 resource at L3 scope

Associating the monitor events with the L3 resource was an implementation choice
that was convenient at the time. Since L2 can similarly be used interchangeably for resource
and scope associating the L2 scope monitoring with the L2 resource may similarly be convenient
when considering how to support L2 monitors as is done in this series.

Even so, I do not believe that associating monitoring with the resource will be
able to cleanly support all MPAM monitoring capability. Specifically the upcoming CPU-less
memory bandwidth monitoring.

As I see the plan at this time: CPU-less memory bandwidth allocation will be
backed by a new MB_NODE resource. For these systems the expectation is that their
monitoring data will be exposed to user space via NODE scope and will appear to user space as:

mon_data/
mon_data/mon_NODE_00
mon_data/mon_NODE_00/mbm_total_bytes <=== monitoring data associated with MB resource at NODE scope

The cover-letter is titled "Generalize monitor events across resources". As I see it such
generalization does not prepare resctrl for all MPAM monitoring capabilities. Could this
work do so?

Reinette