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

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Date:   Thu, 22 Jun 2023 11:09:05 +0530
From:   Ravi Bangoria <ravi.bangoria@....com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     peterz@...radead.org, acme@...nel.org, jolsa@...nel.org,
        namhyung@...nel.org, bp@...en8.de, x86@...nel.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        sandipan.das@....com, ananth.narayan@....com,
        santosh.shukla@....com, Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [PATCH] perf/amd: Prevent grouping of IBS events

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