Re: [PATCH] perf/amd: Prevent grouping of IBS events

[Ravi Bangoria]

Hi Ian,

>> Before:
>>   $ sudo ./perf record -e "{ibs_op//,ibs_op//}" -C 0
>>   ^C[ perf record: Woken up 1 times to write data ]
>>   [ perf record: Captured and wrote 0.540 MB perf.data (531 samples) ]
>>
>> After:
>>   $ sudo ./perf record -e "{ibs_op//,ibs_op//}" -C 0
>>   Error:
>>   AMD IBS may only be available in system-wide/per-cpu mode.
>>   Try using -a, or -C and workload affinity
>>
>> The error message is stale and misleading. I have a patch to fix it.
>> I'll post it separately.
> 
> Thanks Ravi, so this is a workaround for a PMU driver bug where the
> perf_event_open should have failed for the sibling event?

This is not a workaround. This kernel patch fixes PMU driver bug. With
the patch, perf_event_open() will fail for sibling IBS event if either
group leader or any other sibling is of the same IBS pmu. Or did I
misread your comment?

> 
> The behavior is somewhat reminiscent of arch_evsel__must_be_in_group:
> https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/arch/x86/util/evsel.c?h=perf-tools-next#n41
> 
> Normally software events would be valid in the group, should the code
> ignore these?

Grouping of SW and IBS event will continue to work after this patch.

Thanks,
Ravi