perf tests offcpu: Adjust test case perf record offcpu profiling tests for s390

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>
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Co-developed-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20231106091627.2022530-1-tmricht@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files 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