linux-kernel - Re: [PATCH] perf test: Remove atomics from test

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Message-ID: <f0ac7523-edce-4b0b-a142-14c03c912720@arm.com>
Date:   Sat, 25 Nov 2023 19:10:25 +0000
From:   Nick Forrington <nick.forrington@....com>
To:     Leo Yan <leo.yan@...aro.org>, Michael Petlan <mpetlan@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        vmolnaro@...hat.com
Subject: Re: [PATCH] perf test: Remove atomics from test_loop to avoid test
 failures


On 25/11/2023 03:05, Leo Yan wrote:
> Hi all,
>
> On Fri, Nov 24, 2023 at 08:57:52PM +0100, Michael Petlan wrote:
>> On Thu, 2 Nov 2023, Nick Forrington wrote:
>>> The current use of atomics can lead to test failures, as tests (such as
>>> tests/shell/record.sh) search for samples with "test_loop" as the
>>> top-most stack frame, but find frames related to the atomic operation
>>> (e.g. __aarch64_ldadd4_relax).
> I am confused by above description.  As I went through the script
> record.sh, which is the only test invoking the program 'test_loop',
> but I don't find any test is related with stack frame.
>
> Do I miss anything?  I went through record.sh but no clue why the
> failure is caused by stack frame.  All the testings use command:
>
>    if ! perf report -i "${perfdata}" -q | grep -q "${testsym}"
>      ...
>    fi
>
> @Nick, could you narrow down which specific test case causing the
> failure.
>
> [...]


All checks for ${testsym} in record.sh (including the example you 
provide) can fail, as the expected symbol (test_loop) is not the 
top-most function on the stack (and therefore not the symbol associated 
with the sample).


Example perf report output:

# Overhead  Command  Shared Object          Symbol
# ........  .......  ..................... .............................
#
     99.53%  perf     perf                   [.] __aarch64_ldadd4_relax

...


You can see the issue when recording/reporting with call stacks:

# Children      Self  Command  Shared Object          Symbol
# ........  ........  .......  ..................... 
..........................................................
#
     99.52%    99.52%  perf     perf                   [.] 
__aarch64_ldadd4_relax
             |
             |--49.77%--0xffffb905a5dc
             |          0xffffb8ff0aec
             |          thfunc
             |          test_loop
             |          __aarch64_ldadd4_relax

...

>
>> I believe that it was there to prevent the compiler to optimize the loop
>> out or some reason like that. Hopefully, it will work even without that
>> on all architectures with all compilers that are used for building perf...
> Agreed.
>
> As said above, I'd like to step back a bit for making clear what's the
> exactly failure caused by the program.


I don't think this loop could be sensibly optimised away, as it depends 
on "done", which is defined at file scope (and assigned by a signal 
handler).


Cheers,
Nick

>
> Thanks,
> Leo
>