Re: [PATCH V7 3/6] perf, x86: handle multiple records in PEBS buffer

Peter Zijlstra
2015-05-05T17:08:35Z

On Tue, May 05, 2015 at 04:30:25PM +0000, Liang, Kan wrote:
> > > + for (at = base; at < top; at += x86_pmu.pebs_record_size) {
> > > struct pebs_record_nhm *p = at;
> > >
> > > for_each_set_bit(bit, (unsigned long *)&p->status,
> > > x86_pmu.max_pebs_events) {
> > > event = cpuc->events[bit];
> > > WARN_ON_ONCE(!event);
> > >
> > > + if (event->attr.precise_ip)
> > > + break;
> > > + }
> >
> > Would it make sense to delay looking for the event until you've found
> > there is a single bit set -- and already know which bit that is?
> >
>
> Yes, I think we can test cpuc->pebs_enabled here.
> It should be better than attr.precise_ip checking.
>
> - for (; at < top; at += x86_pmu.pebs_record_size) {
> + for (at = base; at < top; at += x86_pmu.pebs_record_size) {
> struct pebs_record_nhm *p = at;
>
> for_each_set_bit(bit, (unsigned long *)&p->status,
> x86_pmu.max_pebs_events) {
> - event = cpuc->events[bit];
> - if (!test_bit(bit, cpuc->active_mask))
> - continue;
> -
> - WARN_ON_ONCE(!event);
>
> - if (!event->attr.precise_ip)
> - continue;
> + if (test_bit(bit, cpuc->pebs_enabled))
> + break;
> + }
>

Can't we take that entire for_each_set_bit() loop out?

It appears to me we effectively do that single test_bit() test you left
in there already with the & cpuc->pebs_enabled later on.

>
> + for (bit = 0; bit < x86_pmu.max_pebs_events; bit++) {
> + if (counts[bit] == 0)
> continue;
> -
> - __intel_pmu_pebs_event(event, iregs, at);
> + event = cpuc->events[bit];
> + WARN_ON_ONCE(!event);
> + WARN_ON_ONCE(!event->attr.precise_ip);
> + __intel_pmu_pebs_event(event, iregs, base,
> + top, bit, counts[bit]);
> }

Right bit of paranoia :-)