Re: [RFC PATCH 0/7] arm_mpam: Add L2 cache monitors

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

Hi Jinjie,

On 22/10/2026 09:48, Jinjie Ruan wrote:
> MPAM resource monitors hang off a partitionable resource in an MSC, not
> off a particular cache level. resctrl only knows about the L3 scope, so
> the L2 cache monitors that MPAM platforms implement are discovered by
> the driver but never show up in resctrl.

Yes, it would be good to add support for more monitoring scopes. This is not just relevant for MPAM.
RISC-V can also have monitors at many places in the system and we should coordinate so we have a
uniform solution across architectures.

> 
> An L2 MSC has the same CSU and MBWU counters as an L3 MSC; the cache
> level comes from the ACPI MPAM table's cache locator. So this is mostly
> a resctrl scope problem, not a hardware one.
> 
> Extend resctrl's monitor scope to L2: register the L2 occupancy and
> bandwidth events against the L2 resource, pick the L2 classes for them,
> and borrow a CSU/MBWU monitor for the duration of a read. With CDP the

Borrowing an MBWU monitor doesn't sound right. The MBWU monitor needs to be configured while the
traffic is happening so that it can keep count of bytes. The CSU is different as it can measure the
occupancy without having to keep any state.

Thanks,

Ben

> two PARTIDs need separate accumulating monitors, so the borrowed-monitor
> context becomes one monitor per PARTID.
> 
> Patches:
> 
>   1-2  Generalise the per-domain read path, add l2c_occupancy and
>        mbm_core_bytes, update the documentation.
> 
>   3    Split the borrowed-monitor context into data and code entries.
> 
>   4    Do the CDP num_rmid halving for every mon_capable resource.
> 
>   5-7  Pick the L2 classes, describe the events with the L2 resource,
>        add the read paths.
> 
> Tested on QEMU with an L2/L3/memory-controller MSC topology. The L2_MON
> files show up and are readable; l2c_occupancy reports the modelled
> occupancy, and mbm_core_bytes reports the modelled bandwidth.
> 
> QEMU branch: https://github.com/ruanjinjie-eng/qemu/tree/mpam
> Test cases: https://github.com/ruanjinjie-eng/mpam_testcase
> 
> Jinjie Ruan (7):
>   fs/resctrl: Generalize __l3_mon_event_count for non-L3 resources
>   fs/resctrl: Add the L2 monitor events
>   arm_mpam: resctrl: Split monitor context for code/data
>   arm_mpam: resctrl: Halve num_rmid for every mon_capable resource on
>     CDP
>   arm_mpam: resctrl: Describe the L2 monitor events with the L2 resource
>   arm_mpam: resctrl: Pick L2 classes for the L2 monitor events
>   arm_mpam: resctrl: Add the L2 monitor read paths
> 
>  Documentation/arch/arm64/mpam.rst |  23 +--
>  drivers/resctrl/mpam_resctrl.c    | 236 +++++++++++++++++++++++-------
>  fs/resctrl/monitor.c              |  15 +-
>  include/linux/resctrl_types.h     |  13 +-
>  4 files changed, 222 insertions(+), 65 deletions(-)
>