perf test: Stat output per thread of just the parent process

Per-thread mode requires either system-wide (-a), a pid (-p) or a tid
(-t).

The stat output tests were using system-wide mode but this is racy when
threads are starting and exiting - something that happens a lot when
running the tests in parallel (perf test -p).

Avoid the race conditions by using pid mode with the pid of the parent
process.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Disha Goel <disgoel@linux.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Link: https://lore.kernel.org/r/20240301074639.2260708-2-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/shell/lib/stat_output.sh

@@ -79,7 +79,7 @@ check_per_thread()
 		echo "[Skip] paranoid and not root"
 		return
 	fi
-	perf stat --per-thread -a $2 true
+	perf stat --per-thread -p $$ $2 true
 	commachecker --per-thread
 	echo "[Success]"
 }

tools/perf/tests/shell/stat+json_output.sh

@@ -105,7 +105,7 @@ check_per_thread()
 		echo "[Skip] paranoia and not root"
 		return
 	fi
-	perf stat -j --per-thread -a -o "${stat_output}" true
+	perf stat -j --per-thread -p $$ -o "${stat_output}" true
 	$PYTHON $pythonchecker --per-thread --file "${stat_output}"
 	echo "[Success]"
 }