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

[Borislav Petkov]

On Fri, Jul 17, 2026 at 04:13:55PM -0500, 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.
> 
> Avoid this compatibility issue by leaving mbm_assign_mode in the "default"
> mode during initialization.

Question: why doesn't the default mode suffer from this counter exhaustion? Or
if it does, why is the whole issue better in the default mode?

Thx.

-- 
Regards/Gruss,
    Boris.

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