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

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Date:   Fri, 14 Jul 2017 14:25:56 -0400 (EDT)
From:   Vince Weaver <vincent.weaver@...ne.edu>
To:     linux-kernel@...r.kernel.org
cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        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

On Fri, 14 Jul 2017, Vince Weaver wrote:

> 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

additional relevant detail:
	in the failing case, the group leader of the event set has 
		.pinned=1
	If I change that to .pinned=0 then the test passes.

Vince