Re: [PATCH 02/10] perf/x86: Improve HT workaround GP counter constraint

[Peter Zijlstra]

On Fri, May 22, 2015 at 05:55:32AM -0700, Stephane Eranian wrote:
> On Fri, May 22, 2015 at 5:53 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Fri, May 22, 2015 at 05:35:14AM -0700, Stephane Eranian wrote:
> >> On Fri, May 22, 2015 at 4:28 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >> > On Fri, May 22, 2015 at 01:21:46PM +0200, Peter Zijlstra wrote:
> >> >> @@ -821,8 +828,24 @@ int x86_schedule_events(struct cpu_hw_ev
> >> >>
> >> >>       /* slow path */
> >> >>       if (i != n) {
> >> >> +             int gpmax = x86_pmu.num_counters / 2;
> >> >> +
> >> >> +             /*
> >> >> +              * Do not allow scheduling of more than half the available
> >> >> +              * generic counters.
> >> >> +              *
> >> >> +              * This helps avoid counter starvation of sibling thread by
> >> >> +              * ensuring at most half the counters cannot be in exclusive
> >> >> +              * mode. There is no designated counters for the limits. Any
> >> >> +              * N/2 counters can be used. This helps with events with
> >> >> +              * specific counter constraints.
> >> >> +              */
> >> >> +             if (is_ht_workaround_enabled() && !cpuc->is_fake &&
> >> >> +                 READ_ONCE(cpuc->excl_cntrs->exclusive_present))
> >> >> +                     gpmax /= 2;
> >> >> +
> >> >>               unsched = perf_assign_events(cpuc->event_constraint, n, wmin,
> >> >> -                                          wmax, assign);
> >> >> +                                          wmax, gpmax, assign);
> >> >>       }
> >> >>
> >> >
> >> > Hmm, I divide by 2 twice.. no wonder it doesn't quite work as expected.
> >>
> >> Yes, that's what I said. Other problem is, with no watchdog, measuring
> >> a non-corrupting event is still multiplexing when more than 2 instances
> >> are passed:
> >>  $ perf stat -a -C 0 -e r20cc,r20cc,r20cc,r20cc -I 1000 sleep 100
> >>
> >> I get 50% scheduling, only 2 out of 4 events scheduled at any time.
> >>
> >> There is nothing running on the sibling thread, so it should let me run with 4
> >> instances as per your patch.
> >
> > Ah, I limited it to n/2 if either of the siblings has an exclusive event
> > on.
> >
> But in my test case above, there was no exclusive event at all on either
> sibling and yet it limited the non-excl to 2.

I bet you tested the exclusive events earlier :-) Its one of the bugs,
the n_excl accounting is leaking up. Once !0 it stays !0.
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