linux-kernel - Re: perf: bisected sampling bug in Linux 4.11-rc1

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Date:   Fri, 14 Jul 2017 23:07:48 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Vince Weaver <vincent.weaver@...ne.edu>,
        linux-kernel@...r.kernel.org
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Stephane Eranian <eranian@...il.com>
Subject: Re: perf: bisected sampling bug in Linux 4.11-rc1

Vince Weaver <vincent.weaver@...ne.edu> writes:

> I was tracking down some regressions in my perf_event_test testsuite.
> Some of the tests broke in the 4.11-rc1 timeframe.
>
> I've bisected one of them, this report is about
> 	tests/overflow/simul_oneshot_group_overflow
> This test creates an event group containing two sampling events, set
> to overflow to a signal handler (which disables and then refreshes the 
> event).
>
> On a good kernel you get the following:
> 	Event perf::instructions with period 1000000
> 	Event perf::instructions with period 2000000
> 		fd 3 overflows: 946 (perf::instructions/1000000)
> 		fd 4 overflows: 473 (perf::instructions/2000000)
> 	Ending counts:
> 		Count 0: 946379875
> 		Count 1: 946365218
>
> With the broken kernels you get:
> 	Event perf::instructions with period 1000000
> 	Event perf::instructions with period 2000000
> 		fd 3 overflows: 938 (perf::instructions/1000000)
> 		fd 4 overflows: 318 (perf::instructions/2000000)
> 	Ending counts:
> 		Count 0: 946373080
> 		Count 1: 653373058

I'm not sure I'm seeing it (granted, it's a friday evening): is it the
difference in overflow counts?

Also, are they cpu or task bound?

Regards,
--
Alex