linux-kernel - Re: [PATCH v6 10/12] perf tools: Improve IBS error handling

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Message-ID: <CABPqkBQZRVqWPFG0Dvc6HGxNcDDb0raaRw4Ur27CvR5O8vwALg@mail.gmail.com>
Date:   Tue, 15 Mar 2022 17:03:45 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     Ravi Bangoria <ravi.bangoria@....com>
Cc:     peterz@...radead.org, kim.phillips@....com, acme@...hat.com,
        jolsa@...hat.com, songliubraving@...com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 10/12] perf tools: Improve IBS error handling

On Tue, Mar 15, 2022 at 12:46 AM Ravi Bangoria <ravi.bangoria@....com> wrote:
>
> Stephane,
>
> On 09-Feb-22 2:46 AM, Stephane Eranian wrote:
> > From: Kim Phillips <kim.phillips@....com>
> >
> > improve the error message returned on failed perf_event_open() on AMD when
> > using IBS.
> >
> > Output of executing 'perf record -e ibs_op// true' BEFORE this patch:
> >
> > The sys_perf_event_open() syscall returned with 22 (Invalid argument)for event (ibs_op//u).
> > /bin/dmesg | grep -i perf may provide additional information.
> >
> > Output after:
> >
> > AMD IBS cannot exclude kernel events.  Try running at a higher privilege level.
> >
> > Output of executing 'sudo perf record -e ibs_op// true' BEFORE this patch:
> >
> > Error:
> > The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (ibs_op//).
> > /bin/dmesg | grep -i perf may provide additional information.
> >
> > Output after:
> >
> > Error:
> > AMD IBS may only be available in system-wide/per-cpu mode.  Try using -a, or -C and workload affinity
>
> This patch seems to be causing regression to perf python test.
>
> Without this patch:
>
>   $ sudo ./perf test -vvv 19
>    19: 'import perf' in python                                         :
>   --- start ---
>   test child forked, pid 145391
>   python usage test: "echo "import sys ; sys.path.append('python'); import perf" | '/usr/bin/python2' "
>   test child finished with 0
>   ---- end ----
>   'import perf' in python: Ok
>
> With this patch:
>
>   $ sudo ./perf test -vvv 19
>    19: 'import perf' in python                                         :
>   --- start ---
>   test child forked, pid 144415
>   python usage test: "echo "import sys ; sys.path.append('python'); import perf" | '/usr/bin/python2' "
>   Traceback (most recent call last):
>     File "<stdin>", line 1, in <module>
>   ImportError: python/perf.so: undefined symbol: perf_env__cpuid
>   test child finished with -1
>   ---- end ----
>   'import perf' in python: FAILED!
>

The fix I sent you is just to prevent a potential SEGFAULT in
is_amd_ibs(). I bet the test fails some perf_event_open()
and ends up in strerror function and from there I don't see how the
patch could impact the test, given you'd segfault
otherwise.

I tried on my side and with or without this patch this test fails. I
think this looks like an unrelated issue.