Re: [PATCH v2 4/6] fs/resctrl: Disallow the software controller when mbm counters are assignable

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

Hi Reinette,

On 3/18/26 15:44, Ben Horgan wrote:

> 
> Apologies if the commit message aren't clear. I do see that I was confused in this one between
> counters being assignable, i.e. mbm_event mode possible, and the user being the being able to
> assign and unassign counters, i.e. mbm_event mode enabled. Do you prefer this as a commit message?
> 
> fs/resctrl: Disallow the software controller when using mbm_event mode
> 
> The software controller requires that there is one MBM counter, for each
> domain, per monitor group that is assigned to the event backing the
> software controller (the mba_MBps_event). When mbm_event mode is in use
> this event backing cannot be guaranteed as there may be insufficient
> counters, the user may unassign the counter or mbm_assign_on_mkdir may be
> set to 0.

Another case which could be mentioned here and  results in unassigned events
is when mbm_assign_on_mkdir is set to 1 but one of the domains was offline
when the monitor group was created.

> 
> Currently, only AMD systems support counter assignment but the MBA is non
> linear and so the software controller is never supported anyway. For MPAM
> systems the MBA is linear and it could support the software controller.
> Adding support would require guaranteeing that the events backing the
> software controller are always assigned. In order to support mbm_event mode
> for MPAM, which is needed to support memory bandwidth monitors, MBM, on
> platforms with fewer bandwidth counters (per domain) than monitor groups,
> take the pragmatic approach and fail the mount if the user requests the
> software controller, the mba_MBps option, when the mbm_event mode is
> enabled. Furthermore, disable switching to mbm_event mode when the software
> controller is enabled. If, in the future, a way for mbm_event mode and the
> software controller to function at the same time is introduced these
> restrictions can be adapted.
Thanks,

Ben