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SubjectRe: [PATCH 02/10] perf/x86: Improve HT workaround GP counter constraint
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@infradead.org> 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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