linux-kernel - Re: [PATCH v3 45/47] arm_mpam: Add workaround for T241-MPAM-4

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Message-ID: <a069fd6f-41d8-45b4-b63d-ccace1a0f501@arm.com>
Date: Fri, 30 Jan 2026 12:21:42 +0000
From: Ben Horgan <ben.horgan@....com>
To: Fenghua Yu <fenghuay@...dia.com>
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
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Subject: Re: [PATCH v3 45/47] arm_mpam: Add workaround for T241-MPAM-4

Hi Fenghua,

On 1/29/26 22:14, Fenghua Yu wrote:
> Hi, Ben,
> 
> On 1/19/26 12:56, Ben Horgan wrote:
>> Hi Fenghua,
>>
>> On 1/15/26 23:20, Fenghua Yu wrote:
>>> Hi, Shanker and Ben,
>>>
>>> On 1/12/26 08:59, Ben Horgan wrote:
>>>> From: Shanker Donthineni <sdonthineni@...dia.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.
>>>
>>> When MBW_MIN=1, min mem bw can be very low when contention. This may
>>> drop mem access performance. Is it possible to set MBW_MIN bigger so
>>> that ensure the floor of mem access is high?
>>
>> Isn't that a policy decision rather than something we should be putting
>> in a quirk framework?
> 
> MBW_MIN is 1% or 5% less than MBW_MAX.
> 
> The lower MBW_MIN hints hardware to lower mem bandwidth when mem access
> contention. That causes memory performance degradation.
> 
> Is it possible to do the following changes to fix the performance issue?
> 1. By default min mbw is equal to max mbw. So hardware won't lower
> performance unless it's needed. This can fix the current performance issue.
> 2. Add a new schemata line (e.g. MBI:<id>=x;<id>=y;...) to specify min
> mbw just like max mbw specified by schemata line "MB:...". User can use
> this line to change min mbw per partition per node. This could be added
> in the future.

Thanks for bringing this up. It raises some more general queries about
the handling of mbw_min and so I'll move the discussion to
[PATCH v3 41/47] arm_mpam: Generate a configuration for min controls
as it isn't specific to the nvidia quirks.

> 
> Thanks.
> 
> -Fenghua

Thanks,

Ben