AMD Posts Linux Patches For QoS RMID Pinning / ABMC

One of the latest Linux kernel patch series posted by AMD Linux engineers is for enabling an AMD QoS RMID pinning feature found within their latest generation processors.

AMD QoS RMID Pinning is perhaps better known and more descriptive by its other name, Assignable Bandwidth Monitoring Counters (ABMC). The new AMD kernel patches adding this ABMC integration to the resource control "resctrl" code explains:

"AMD hardware can support 256 or more RMIDs. However, bandwidth monitoring feature only guarantees that RMIDs currently assigned to a processor will be tracked by hardware. The counters of any other RMIDs which are no longer being tracked will be reset to zero. The MBM event counters return "Unavailable" for the RMIDs that are not active.

Users can create 256 or more monitor groups. But there can be only limited number of groups that can be give guaranteed monitoring numbers. With ever changing system configuration, there is no way to definitely know which of these groups will be active for certain point of time. Users do not have the option to monitor a group or set of groups for certain period of time without worrying about RMID being reset in between.

The ABMC feature provides an option to pin (or assign) the RMID to the hardware counter and monitor the bandwidth for a longer duration. The pinned RMID will be active until the user unpins (or unassigns) it. There is no need to worry about counters being reset during this period. Additionally, the user can specify a bitmask identifying the specific bandwidth types from the given source to track with the counter."

These 15 patches now out for review allow exposing this ABMC support under Linux. Via the /sys/fs/resctrl interfaces is then support for user-space to make use of these Assignable Bandwidth Monitoring Counters for various quality of service monitoring purposes.

This feature will mainly be of interest for server administrators and those profiling AMD EPYC servers for performance bottlenecks.