hwpmc: fix performance issues

[wma]

Summary

Avoid using atomics as it_wait is guarded by td_lock.

Report threshold calculation is done only if at least one PMC hook
is installed

• there will be another patch with pmc.soft(3) update

[mjg]

You should always modify the conter, but only do branch on stuff depending on PMC_HOOK_INSTALLED_ANY

[mjg]

sys/kern/kern_intr.c

1027–1030

the point was to avoid needing to read intr_hwpmc_waiting_report_threshold if hwpmc is not loaded. So this should be:

it->it_waiting++; 
if (PMC_HOOK_INSTALLED_ANY()) {
        if (it->it_waiting >= intr_hwpmc_waiting_report_threshold) {
                PMC_SOFT_CALL_INTR_HLPR(waiting, frame);
        }
}.

or so

[wma]

OK, so if I got your point, you meant

• avoid unnecessary branching (if frame != null ...) by having PMC_HOOK_INSTALLED_ANY condition on the top of them, which should hint the core not to execute speculatively anything which us underneath;
• access intr_hwpmc_waiting_report_threshold cacheline only if at least one hook is loaded;

right?

[mjg]

yea