Re: [PATCH v6 19/22] x86/resctrl: Introduce the interface to switch between monitor modes

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Peter,

On 8/16/24 10:16 AM, Peter Newman wrote:
> Hi Reinette,
> 
> On Fri, Aug 16, 2024 at 10:01 AM Reinette Chatre
> <reinette.chatre@intel.com> wrote:
>>
>> Hi James,
>>
>> On 8/16/24 9:31 AM, James Morse wrote:
>>> Hi Babu,
>>>
>>> On 06/08/2024 23:00, Babu Moger wrote:
>>>> Introduce interface to switch between ABMC and legacy modes.
>>>>
>>>> By default ABMC is enabled on boot if the feature is available.
>>>> Provide the interface to go back to legacy mode if required.
>>>
>>> I may have missed it on an earlier version ... why would anyone want the non-ABMC
>>> behaviour on hardware that requires it: counters randomly reset and randomly return
>>> 'Unavailable'... is that actually useful?
>>>
>>> You default this to on, so there isn't a backward compatibility argument here.
>>>
>>> It seems like being able to disable this is a source of complexity - is it needed?
>>
>> The ability to go back to legacy was added while looking ahead to support the next
>> "assignable counter" feature that is software based ("soft-RMID" .. "soft-ABMC"?).
>>
>> This series adds support for ABMC on recent AMD hardware to address the issue described
>> in cover letter. This issue also exists on earlier AMD hardware that does not have the ABMC
>> feature and Peter is working on a software solution to address the issue on non-ABMC hardware.
>> This software solution is expected to have the same interface as the hardware solution but
>> earlier discussions revealed that it may introduce extra latency that users may only want to
>> accept during periods of active monitoring. Thus the option to disable the counter assignment
>> mode.
> 
> Sorry again for the soft-RMID/soft-ABMC confusion[1], it was soft-RMID
> that impacted context switch latency. Soft-ABMC does not require any
> additional work at context switch.

No problem. I did read [1] but I do not think I've seen soft-ABMC yet so
my understanding of what it does is vague.

> The only disadvantage to soft-ABMC I can think of is that it also
> limits reading llc_occupancy event counts to "assigned" groups,
> whereas without it, llc_occupancy works reliably on all RMIDs on AMD
> hardware.

hmmm ... keeping original llc_occupancy behavior does seem useful enough
as motivation to keep the "legacy"/"default" mbm_assign_mode? It does sound
to me as though soft-ABMC may not be as accurate when it comes to llc_occupancy.
As I understand the hardware may tag entries in cache with RMID and that has a longer
lifetime than the tasks that allocated that data into the cache. If soft-ABMC
permanently associates an RMID with a local and total counter pair but that
RMID is dynamically assigned to resctrl groups then a group may not always
get the same RMID ... and thus its llc_occupancy data would be a combination of
its cache allocations and all the cache allocations of resource groups that had
that RMID before it. This may need significantly enhanced "limbo" handling?

Reinette