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

Ben Horgan
2026-02-18T16:22:17Z

Hi Fenghua, Zeng,

On 2/16/26 13:54, Ben Horgan wrote:
> 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

Fenghua gave a talk at LPC on supporting cpu-less numa nodes in resctrl
so is likely to have some patches/ideas around measuring bandwidth at
memory controllers.

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

Thanks,

Ben