linux-kernel - Re: perf tool: About "perf arm64: Inject missing frames when using 'perf record --call-graph=fp'"

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Date:   Wed, 6 Apr 2022 12:28:55 +0100
From:   John Garry <john.garry@...wei.com>
To:     James Clark <james.clark@....com>, <alexandre.truong@....com>
CC:     <linux-perf-users@...r.kernel.org>,
        "jolsa@...nel.org >> Jiri Olsa" <jolsa@...nel.org>,
        <german.gomez@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: perf tool: About "perf arm64: Inject missing frames when using
 'perf record --call-graph=fp'"

On 06/04/2022 10:08, James Clark wrote:
> 
> 
> On 05/04/2022 15:04, John Garry wrote:
>> Hi Alexandre,
>>
>> I notice that with commit b9f6fbb3b2c2 ("perf arm64: Inject missing frames when using 'perf record --call-graph=fp'") that I get messages spewing the console when running perf record+report, as below:
>>
>> john@...ntu:~/linux$sudo tools/perf/perf record -ag fio null12.fio
>> john@...ntu:~/linux$sudo tools/perf/perf report > report
>> unwind: can't read reg 29
>> unwind: can't read reg 29
>> unwind: can't read reg 29
>> unwind: can't read reg 29
>> ...
>>
>> Do you know the possible cause? I haven't checked...
> 
> Hi John,
> 
> I'm going to look into this today.

Great

  I expect the cause is because we only record
>> the link register for this change and then do a best effort unwind to see if
> we can get the return address just from that. So I don't think this is a major issue,
> probably the outcome will be that I mask any of these errors just for this call
> to libunwind that we added. The other main call to libunwind should still print
> these errors.
> 
> One thing that is interesting is why we didn't see this when we were testing
> the patch before, and we've also found it a little bit difficult to reproduce here.
> So there might be more to it than just masking the error, but I'm not sure yet.
> 
> Either way, I don't expect that any unwinding is broken, just that it's
> printing an annoying message.

Yeah, the actual ouput looks ok at a glance. I really didn't check much.

Thanks,
John