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SubjectRe: [PATCH] perf/tests: Fix test case 95 on s390 and use same event
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Thanks for the fix, Thomas

On 27/07/2022 15:14, Thomas Richter wrote:
> On linux-next tree perf test 95 was added recently.
> s390 does not support branch sampling at all and the test case fails
> despite for checking branch support before hand.
> The check for support of branching
> uses the software event named dummy, as seen in the line:
>
> perf record -b -o- -e dummy -B true > /dev/null 2>&1 || exit 2

Just curious, do you know why the command succeeds in this platform (and
potentially others)?

I got the idea of using "dummy" from [1] but only tested on arm64 and
x86. I thought the platforms would reject it if -b was not implemented
regardless of the event. Did I misunderstand the use of dummy?

Thanks,
German

[1] https://lore.kernel.org/all/20220617073840.GA45710@leoy-ThinkPad-X240s/

>
> However when the branch recording is actually done, a different
> event is used, as seen in the line:
>
> perf record -o $TMPDIR/... --branch-filter any,save_type,u -- ...
>
> The event is omitted and for perf record the default event is
> cycles, which is not supported by s390 and this fails when executed
> on s390:
>
> # perf record --branch-filter any,save_type,u -- \
> /tmp/__perf_test.program.iDSmQ/a.out
> Error:
> cycles: PMU Hardware or event type doesn't support branch stack sampling.
> #
>
> Therefore fix this and use the same event cycles for testing support
> and actually running the test.
>
> Output before:
> # ./perf test -Fv 95
> 95: Check branch stack sampling :
> --- start ---
> Testing user branch stack sampling
> ---- end ----
> Check branch stack sampling: FAILED!
> #
>
> Output after:
> # ./perf test -Fv 95
> 95: Check branch stack sampling :
> --- start ---
> ---- end ----
> Check branch stack sampling: Skip
> #
>
> Fixes: b55878c90ab9 ("perf test: Add test for branch stack sampling")
> Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
> ---
> tools/perf/tests/shell/test_brstack.sh | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/perf/tests/shell/test_brstack.sh b/tools/perf/tests/shell/test_brstack.sh
> index 113ccd17bf03..c644f94a6500 100755
> --- a/tools/perf/tests/shell/test_brstack.sh
> +++ b/tools/perf/tests/shell/test_brstack.sh
> @@ -12,7 +12,7 @@ if ! [ -x "$(command -v cc)" ]; then
> fi
>
> # skip the test if the hardware doesn't support branch stack sampling
> -perf record -b -o- -e dummy -B true > /dev/null 2>&1 || exit 2
> +perf record -b -o- -B true > /dev/null 2>&1 || exit 2
>
> TMPDIR=$(mktemp -d /tmp/__perf_test.program.XXXXX)
>

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