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

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Message-ID: <d32a5a9f-f634-ea3e-d68e-b07c9bab9add@amd.com>
Date:   Wed, 16 Mar 2022 16:37:06 +0530
From:   Ravi Bangoria <ravi.bangoria@....com>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     peterz@...radead.org, kim.phillips@....com, acme@...hat.com,
        jolsa@...hat.com, songliubraving@...com,
        linux-kernel@...r.kernel.org, Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [PATCH v6 10/12] perf tools: Improve IBS error handling

Stephane,

On 16-Mar-22 5:33 AM, Stephane Eranian wrote:
> 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.

That's strange. IIUC, python/perf.so needs to know about util/evsel.c
which can be done by adding an entry in util/python-ext-sources.

In any case, do we really need this patch now? For both the example given
in description, I see no difference with or without patch:

Without patch:

  $ sudo ./perf record -e ibs_op// true
  Error:
  Invalid event (ibs_op//) in per-thread mode, enable system wide with '-a'.

  $ ./perf record -e ibs_op// true
  Error:
  Invalid event (ibs_op//u) in per-thread mode, enable system wide with '-a'.

Same o/p with the patch as well.

Thanks,
Ravi