ftrace/selftests: workaround cgroup RT scheduling issues

wakeup_rt.tc and wakeup.tc tests in tracers/ subdirectory
fail due to the chrt command returning:

 chrt: failed to set pid 0's policy: Operation not permitted.

To work around this, temporarily disable grout RT scheduling
during ftracetest execution.  Restore original value on
test run completion.  With these changes in place, both
tests consistently pass.

Fixes: c575dea2c1 ("selftests/ftrace: Add wakeup_rt tracer testcase")
Fixes: c1edd060b4 ("selftests/ftrace: Add wakeup tracer testcase")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/ftrace/ftracetest

@@ -29,8 +29,25 @@ err_ret=1
 # kselftest skip code is 4
 err_skip=4
 
+# cgroup RT scheduling prevents chrt commands from succeeding, which
+# induces failures in test wakeup tests.  Disable for the duration of
+# the tests.
+
+readonly sched_rt_runtime=/proc/sys/kernel/sched_rt_runtime_us
+
+sched_rt_runtime_orig=$(cat $sched_rt_runtime)
+
+setup() {
+  echo -1 > $sched_rt_runtime
+}
+
+cleanup() {
+  echo $sched_rt_runtime_orig > $sched_rt_runtime
+}
+
 errexit() { # message
   echo "Error: $1" 1>&2
+  cleanup
   exit $err_ret
 }
 
@@ -39,6 +56,8 @@ if [ `id -u` -ne 0 ]; then
   errexit "this must be run by root user"
 fi
 
+setup
+
 # Utilities
 absdir() { # file_path
   (cd `dirname $1`; pwd)
@@ -235,6 +254,7 @@ TOTAL_RESULT=0
 
 INSTANCE=
 CASENO=0
+
 testcase() { # testfile
   CASENO=$((CASENO+1))
   desc=`grep "^#[ \t]*description:" $1 | cut -f2 -d:`
@@ -406,5 +426,7 @@ prlog "# of unsupported: " `echo $UNSUPPORTED_CASES | wc -w`
 prlog "# of xfailed: " `echo $XFAILED_CASES | wc -w`
 prlog "# of undefined(test bug): " `echo $UNDEFINED_CASES | wc -w`
 
+cleanup
+
 # if no error, return 0
 exit $TOTAL_RESULT