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

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Date:	Fri, 22 May 2015 05:55:32 -0700
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Jiri Olsa <jolsa@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/10] perf/x86: Improve HT workaround GP counter constraint

On Fri, May 22, 2015 at 5:53 AM, Peter Zijlstra <peterz@...radead.org> 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@...radead.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.
>
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.

> 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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