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

[Alexander Shishkin]

Vince Weaver <vincent.weaver@xxxxxxxxx> 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
>
>
> 487f05e18aa4efacee6357480f293a5afe6593b5 is the first bad commit
>
> commit 487f05e18aa4efacee6357480f293a5afe6593b5
> Author: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Date:   Thu Jan 19 18:43:30 2017 +0200

Ok, there was a bug there indeed. This patch should take care of it and
should also be backportable in case it's stable-worthy.