Re: [PATCH 1/2] x86/resctrl, Documentation: Keep mbm_assign_mode "default" on boot

[Reinette Chatre]

Hi Babu,

On 7/17/26 2:13 PM, Babu Moger wrote:
> The kernel currently enables the ABMC-based "mbm_event" mode by default on
> hardware that supports it. However, this can cause bandwidth monitoring
> failures with existing userspace tools such as pqos.
> 
> The pqos tool mounts the resctrl filesystem and creates 16 or more resctrl
> groups by default. On systems with 32 or fewer ABMC counters, this default
> configuration can consume all available counters, since each group requires
> one counter for local MBM and another for total MBM. If additional
> monitoring groups are created, counter resources are exhausted and pqos
> tool reports memory bandwidth counters as zero for those groups.

It is not obvious to me that this is a problem. If I understand correctly 
there are two scenarios possible with this pqos behavior:

- ABMC is not in use ("mbm_assign_mode" is set to "default")
  - pqos can create 16 or more monitor groups
  - hardware still supports a limited number of counters with consequence that
    underlying counters reset at any time as the different monitoring groups
    need to be tracked.
  - pqos can read monitoring data of all 16 monitor groups, sometimes reading the
    events would return "Unavailable", sometimes reading the events return data.
  - *None* of the monitoring numbers returned are guaranteed to be accurate.

- ABMC is in use ("mbm_assign_mode" is set to "mbm_event"):
  - pqos can create 16 or more monitor groups
  - only a subset of monitoring groups have counters assigned and these counters
    are guaranteed to only track the monitor groups/events they are assigned to
  - pqos can read monitoring data of all 16 monitor groups with two possibilities:
    - monitor group/event has counter assigned: monitoring numbers are guaranteed to be accurate
    - monitor group/event does not have counter assigned: monitoring numbers return 0

If my understanding is correct then the preference is to rather have wrong data than
see 0? This does not sound right. What am I missing?

The changelog starts with "this can cause bandwidth monitoring failures with existing
userspace tools such as pqos". How could returning accurate memory bandwidth data be
considered a failure? How does this issue manifest itself?

> 
> Avoid this compatibility issue by leaving mbm_assign_mode in the "default"
> mode during initialization. Users who want to use ABMC can continue to
> enable it explicitly:
> 
>  echo mbm_event > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> 
> Update the resctrl documentation to reflect the new boot-time default and
> adjust the mbm_assign_mode examples accordingly.
> 
> Signed-off-by: Babu Moger <babu.moger@xxxxxxx>
> ---
> There are plans to enable "mbm_event" by default once additional counters
> are available. For now, keep the default mode to maintain compatibility
> with existing tools.

This is not ideal. resctrl should aim to provide a consistent user interface
across kernel versions. 

Reinette