selftests/ftrace: Add ringbuffer size changing testcase

Add a testcase for changing ringbuffer size. This tests
not only ringbuffer size but also tests the imbalance
per-cpu buffer size change too.
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>

tools/testing/selftests/ftrace/test.d/00basic/ringbuffer_size.tc

+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# description: Change the ringbuffer size
+# flags: instance
+
+rb_size_test() {
+ORIG=`cat buffer_size_kb`
+
+expr $ORIG / 2 > buffer_size_kb
+
+expr $ORIG \* 2 > buffer_size_kb
+
+echo $ORIG > buffer_size_kb
+}
+
+rb_size_test
+
+: "If per-cpu buffer is supported, imbalance it"
+if [ -d per_cpu/cpu0 ]; then
+  cd per_cpu/cpu0
+  rb_size_test
+fi