Re: [PATCH v4 26/41] arm_mpam: resctrl: Add support for 'MB' resource

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

Hi Zeng,

On 2/13/26 07:38, Zeng Heng wrote:
> Hi Ben,
> 
> On 2026/2/6 0:50, Jonathan Cameron wrote:
>> On Tue, 3 Feb 2026 21:43:27 +0000
>> Ben Horgan <ben.horgan@arm.com> wrote:
>>
>>> From: James Morse <james.morse@arm.com>
>>>
>>> resctrl supports 'MB', as a percentage throttling of traffic from the
>>> L3. This is the control that mba_sc uses, so ideally the class chosen
>>> should be as close as possible to the counters used for mbm_total. If
>>> there is a single L3 and the topology of the memory matches then the
>>> traffic at the memory controller will be equivalent to that at egress of
>>> the L3. If these conditions are met allow the memory class to back MB.
>>>
>>> MB's percentage control should be backed either with the fixed point
>>> fraction MBW_MAX or bandwidth portion bitmaps. The bandwidth portion
>>> bitmaps is not used as its tricky to pick which bits to use to avoid
>>> contention, and may be possible to expose this as something other than a
>>> percentage in the future.
>>
>> I'm very curious to know whether this heuristic is actually useful on
>> many
>> systems or whether many / most of them fail this 'shape' heuristic.
>>
> 
> The current MPAM driver has restrictions that limit MB control support.
> For example, on some systems, the MPAM memory class MSCs are not located
> at the L3 cache but rather at the memory controller (e.g., Mata). In
> such cases, MB control and mbm_total bandwidth monitoring cannot be
> enabled.
> > I'm unsure whether the community would welcome and be willing to review
> a patch series supporting such systems. Of course, the changes would
> involve minor refactoring in the resctrl common layer.

Having MSC at the memory controllers is quite common and it would be
good for the mpam driver and resctrl to support this. My current
priority is the initial MPAM support and look at this and other extra
features later but I'd be happy to help progress support in this area
through review and discussion. There is some discussion about adding new
schema at [1] and we should make sure  this is consistent with monitors
too. James has some out of tree patches from before that disccussion at [2].

[1] https://lore.kernel.org/lkml/aPtfMFfLV1l%2FRB0L@e133380.arm.com/
[2] git://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git
 mpam/snapshot+extras/v6.18-rc1

> 
> Any feedback would be greatly appreciated.
> 
> 
> Best Regards,
> Zeng Heng

Thanks,

Ben