[PATCH v4 38/41] arm_mpam: Add workaround for T241-MPAM-4

[Zeng Heng]

Hi Shaopeng,

On 2026/2/13 15:02, Shaopeng Tan (Fujitsu) wrote:
> Hello Ben, Fenghua
> 
>> From: Shanker Donthineni <sdonthineni at nvidia.com>
>>
>> In the T241 implementation of memory-bandwidth partitioning, in the absence
>> of contention for bandwidth, the minimum bandwidth setting can affect the
>> amount of achieved bandwidth. Specifically, the achieved bandwidth in the
>> absence of contention can settle to any value between the values of
>> MPAMCFG_MBW_MIN and MPAMCFG_MBW_MAX.  Also, if MPAMCFG_MBW_MIN is set
>> zero (below 0.78125%), once a core enters a throttled state, it will never
>> leave that state.
>>
>> The first issue is not a concern if the MPAM software allows to program
>> MPAMCFG_MBW_MIN through the sysfs interface. This patch ensures program
>> MBW_MIN=1 (0.78125%) whenever MPAMCFG_MBW_MIN=0 is programmed.
>>
>> In the scenario where the resctrl doesn't support the MBW_MIN interface via
>> sysfs, to achieve bandwidth closer to MBW_MAX in the absence of contention,
>> software should configure a relatively narrow gap between MBW_MIN and
>> MBW_MAX. The recommendation is to use a 5% gap to mitigate the problem.
> 
> I have a question regarding the MBW_MIN values.
> Are there any cases where the sum of all MBW_MIN values from different control groups exceeds 100%?
> And if so, is it acceptable for it to exceed 100%?"
> 
> Best regards,
> Shaopeng TAN
> 


Per the ARM MPAM architecture specification: "A PARTID that has used
less than MIN is given preferential access to bandwidth."

MBW_MIN is not a guaranteed bandwidth allocation. Instead, it serves as
a priority threshold: when a partid's memory bandwidth usage falls below
the configured MBW_MIN value, its priority for memory bandwidth access
is elevated.

Therefore, it is acceptable for the sum of MBW_MIN values across
different control groups to exceed 100%. There is no requirement
for these values to add up to 100% or less.

Hope this clarifies the behavior.


Best regards,
Zeng Heng