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

Mi, Dapeng
2025-04-16T01:46:59Z

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;
}

>
>> 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
>