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

[Peter Zijlstra]

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.

In any case, there's at least two more bugs in that patch that I know
of, one of which I've fixed, the other I know how to fix.

Will re-post in a bit once I got it tested.
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