Hi Reinette,
On Fri, Aug 16, 2024 at 10:01 AM Reinette Chatre
<reinette.chatre@xxxxxxxxx> 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.
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.
-Peter
[1] https://lore.kernel.org/lkml/CALPaoChDv+irGEmccaQ6SpsuVS8PZ_cfzPgceq3hD3N2cqNjZA@xxxxxxxxxxxxxx/
