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@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.
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
