Message-ID: <6c77b065-a54e-4b9c-a4cf-8b81676f2ab2@amd.com>
Date: Tue, 20 May 2025 12:51:25 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>,
 Peter Newman <peternewman@...gle.com>
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Subject: Re: [PATCH v13 00/27] x86/resctrl : Support AMD Assignable Bandwidth
 Monitoring Counters (ABMC)

Hi Reinette,

On 5/20/25 11:06, Reinette Chatre wrote:
> Hi Babu,
> 
> On 5/20/25 8:28 AM, Moger, Babu wrote:
>> On 5/19/25 10:59, Peter Newman wrote:
>>> On Fri, May 16, 2025 at 12:52 AM Babu Moger <babu.moger@....com> wrote:
> 
> ...
> 
>>>> /sys/fs/resctrl/info/L3_MON/num_mbm_cntrs: Reports the number of monitoring
>>>> counters available for assignment.
>>>
>>> Earlier I discussed with Reinette[1] what num_mbm_cntrs should
>>> represent in a "soft-ABMC" implementation where assignment is
>>> implemented by assigning an RMID, which would result in all events
>>> being assigned at once.
>>>
>>> My main concern is how many "counters" you can assign by assigning
>>> RMIDs. I recall Reinette proposed reporting the number of groups which
>>> can be assigned separately from counters which can be assigned.
>>
>> More context may be needed here. Currently, num_mbm_cntrs indicates the
>> number of counters available per domain, which is 32.
>>
>> At the moment, we can assign 2 counters to each group, meaning each RMID
>> can be associated with 2 hardware counters. In theory, it's possible to
>> assign all 32 hardware counters to a group—allowing one RMID to be linked
>> with up to 32 counters. However, we currently lack the interface to
>> support that level of assignment.
>>
>> For now, the plan is to support basic assignment and expand functionality
>> later once we have the necessary data structure and requirements.
> 
> Looks like some requirements did not make it into this implementation.
> Do you recall the discussion that resulted in you writing [2]? Looks like
> there is a question to Peter in there on how to determine how many "counters"
> are available in soft-ABMC. I interpreted [3] at that time to mean that this
> information would be available in a future AMD publication.

We already have a method to determine the number of counters in soft-ABMC
mode, which Peter has addressed [4].

[4]
https://lore.kernel.org/lkml/20250203132642.2746754-1-peternewman@google.com/

This appears to be more of a workaround, and I doubt it will be included
in any official AMD documentation. Additionally, the long-term direction
is moving towards ABMC.

I don’t believe this workaround needs to be part of the current series. It
can be added later when soft-ABMC is implemented.

> 
> Reinette
> 
> [2] https://lore.kernel.org/lkml/afb99efe-0de2-f7ad-d0b8-f2a0ea998efd@amd.com/ 
> [3] https://lore.kernel.org/lkml/CALPaoCg3KpF94g2MEmfP_Ro2mQZYFA8sKVkmb+7isotKNgdY9A@mail.gmail.com/

-- 
Thanks
Babu Moger

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