Re: [Patch v3 12/22] perf/x86/intel: Update dyn_constranit base on PEBS event precise level

Mi, Dapeng
2025-04-17T01:15:23Z

On 4/16/2025 9:59 PM, Liang, Kan wrote:
>
> On 2025-04-15 9:46 p.m., Mi, Dapeng wrote:
>> On 4/16/2025 12:31 AM, Liang, Kan wrote:
>>> On 2025-04-15 9:53 a.m., Peter Zijlstra wrote:
>>>> On Tue, Apr 15, 2025 at 11:44:18AM +0000, Dapeng Mi wrote:
>>>>> arch-PEBS provides CPUIDs to enumerate which counters support PEBS
>>>>> sampling and precise distribution PEBS sampling. Thus PEBS constraints
>>>>> should be dynamically configured base on these counter and precise
>>>>> distribution bitmap instead of defining them statically.
>>>>>
>>>>> Update event dyn_constraint base on PEBS event precise level.
>>>> What if any constraints are there on this?
>>> Do you mean the static constraints defined in the
>>> event_constraints/pebs_constraints?
>>>
>>>> CPUID is virt host
>>>> controlled, right, so these could be the most horrible masks ever.
>>>>
>>> Yes, it could be changed by VMM. A sanity check should be required if
>>> abad mask is given.
>> Yes, we need a check to restrict the PEBS counter mask into the valid
>> counter mask, and just realized that we can't use hybrid(event->pmu,
>> intel_ctrl) to check counter mask and need a minor tweak since it includes
>> the GLOBAL_CTRL_EN_PERF_METRICS bit.
>>
>> How about this?
>>
>>         if (x86_pmu.arch_pebs) {
>>             u64 cntr_mask = hybrid(event->pmu, intel_ctrl) &
>>                         ~GLOBAL_CTRL_EN_PERF_METRICS;
>>             u64 pebs_mask = event->attr.precise_ip >= 3 ?
>>                         pebs_cap.pdists : pebs_cap.counters;
>>             if (pebs_mask != cntr_mask)
>>                 event->hw.dyn_constraint = pebs_mask & cntr_mask;
>>         }
>>
> The mask isn't changed after boot. The sanity check should only be done
> once. I think it can be done in the update_pmu_cap() when perf retrieves
> the value. If a bad mask is detected, the PEBS should be disabled.

Yeah, it makes sense. Would do.

>
> Thanks,
> Kan>
>>>> This can land us in EVENT_CONSTRAINT_OVERLAP territory, no?The dyn_constraint is a supplement of the static constraints. It doesn't
>>> overwrite the static constraints.
>>>
>>> In the intel_get_event_constraints(), perf always gets the static
>>> constraints first. If the dyn_constraint is defined, it gets the common
>>> mask of the static constraints and the dynamic constraints. All
>>> constraint rules will be complied.
>>>
>>> if (event->hw.dyn_constraint != ~0ULL) {
>>> c2 = dyn_constraint(cpuc, c2, idx);
>>> c2->idxmsk64 &= event->hw.dyn_constraint;
>>> c2->weight = hweight64(c2->idxmsk64);
>>> }
>>>
>>> Thanks,
>>> Kan
>>>
>