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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Mon, 6 Nov 2023 07:21:35 -0800
Message-ID: <CAM9d7chpvhDCkTLiuPVqSKxoZdNYgJ0A2HvVdXbEgMvPZrv1Eg@mail.gmail.com>
Subject: Re: [PATCH v2 PING] perf test: Adjust test case perf record offcpu
 profiling tests for s390
To:     Thomas Richter <tmricht@linux.ibm.com>
Cc:     linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        acme@kernel.org, svens@linux.ibm.com, gor@linux.ibm.com,
        sumanthk@linux.ibm.com, hca@linux.ibm.com,
        Ilya Leoshkevich <iii@linux.ibm.com>
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Hello,

On Mon, Nov 6, 2023 at 1:16 AM Thomas Richter <tmricht@linux.ibm.com> wrote:
>
> On s390 using linux-next the test case
>     87: perf record offcpu profiling tests
> fails. The root cause is this command
>
>  # ./perf  record --off-cpu -e dummy -- ./perf bench sched messaging -l 10
>  # Running 'sched/messaging' benchmark:
>  # 20 sender and receiver processes per group
>  # 10 groups == 400 processes run
>
>      Total time: 0.231 [sec]
>  [ perf record: Woken up 1 times to write data ]
>  [ perf record: Captured and wrote 0.077 MB perf.data (401 samples) ]
>  #
>
> It does not generate 800+ sample entries, on s390 usually around 40[1-9],
> sometimes a few more, but never more than 450. The higher the number
> of CPUs the lower the number of samples.
>
> Looking at function chain
>   bench_sched_messaging()
>   +--> group()
> the senders and receiver threads are created. The senders and receivers
> call function ready() which writes one bytes and wait for a reply using
> poll system() call.
>
> As context switches are counted, the function ready() will trigger
> a context switch when no input data is available after the write
> system call. The write system call does not trigger context switches
> when the data size is small. And writing 1000 bytes (10 iterations with
> 100 bytes) is not much and certainly won't block.
>
> The 400+ context switch on s390 occur when the some receiver/sender
> threads call ready() and wait for the response from function
> bench_sched_messaging() being kicked off.
>
> Lower the number of expected context switches to 400 to succeed on
> s390.
>
> Suggested-by: Namhyung Kim <namhyung@kernel.org>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> Co-developed-by: Ilya Leoshkevich <iii@linux.ibm.com>
> Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>

Acked-by: Namhyung Kim <namhyung@kernel.org>

Thanks,
Namhyung


> ---
>  tools/perf/tests/shell/record_offcpu.sh | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/perf/tests/shell/record_offcpu.sh b/tools/perf/tests/shell/record_offcpu.sh
> index a1ef8f0d2b5c..67c925f3a15a 100755
> --- a/tools/perf/tests/shell/record_offcpu.sh
> +++ b/tools/perf/tests/shell/record_offcpu.sh
> @@ -77,9 +77,9 @@ test_offcpu_child() {
>      err=1
>      return
>    fi
> -  # each process waits for read and write, so it should be more than 800 events
> +  # each process waits at least for poll, so it should be more than 400 events
>    if ! perf report -i ${perfdata} -s comm -q -n -t ';' --percent-limit=90 | \
> -    awk -F ";" '{ if (NF > 3 && int($3) < 800) exit 1; }'
> +    awk -F ";" '{ if (NF > 3 && int($3) < 400) exit 1; }'
>    then
>      echo "Child task off-cpu test [Failed invalid output]"
>      err=1
> --
> 2.41.0
>