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

From: Zeng Heng

Date: Fri Feb 13 2026 - 20:32:58 EST

Hi Shaopeng,

On 2026/2/13 15:02, Shaopeng Tan (Fujitsu) wrote:

Hello Ben, Fenghua

From: Shanker Donthineni <sdonthineni@xxxxxxxxxx>

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