perf tests: Add test for trace output loss

Add a test that checks that trace output is not lost to races. This is
accomplished by tracing the exit_group syscall of "true" multiple times and
checking for correct output.

Signed-off-by: Benjamin Peterson <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Tested-by: Howard Chu <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Kan Liang <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[ Addressed two ShellCheck warnings ]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/tests/shell/trace_exit_race.sh

···
+#!/bin/sh
+# perf trace exit race
+# SPDX-License-Identifier: GPL-2.0
+
+# Check that the last events of a perf trace'd subprocess are not
+# lost. Specifically, trace the exiting syscall of "true" 10 times and ensure
+# the output contains 10 correct lines.
+
+# shellcheck source=lib/probe.sh
+. "$(dirname $0)"/lib/probe.sh
+
+skip_if_no_perf_trace || exit 2
+
+trace_shutdown_race() {
+	for _ in $(seq 10); do
+		perf trace -e syscalls:sys_enter_exit_group true 2>>$file
+	done
+	[ "$(grep -c -E ' +[0-9]+\.[0-9]+ +true/[0-9]+ syscalls:sys_enter_exit_group\(\)$' $file)" = "10" ]
+}
+
+
+file=$(mktemp /tmp/temporary_file.XXXXX)
+
+# Do not use whatever ~/.perfconfig file, it may change the output
+# via trace.{show_timestamp,show_prefix,etc}
+export PERF_CONFIG=/dev/null
+
+trace_shutdown_race
+err=$?
+rm -f ${file}
+exit $err