rcutorture: Allow specifying per-scenario stat_interval

The rcutorture test suite makes double use of the rcutorture.stat_interval
module parameter.  As its name suggests, it controls the frequency
of statistics printing, but it also controls the rcu_torture_writer()
stall timeout.  The current setting of 15 seconds works surprisingly well.
However, given that the RCU tasks stall-warning timeout is ten -minutes-,
15 seconds is too short for TASKS02, which runs a non-preemptible kernel
on a single CPU.

This commit therefore adds checks for per-scenario specification of the
rcutorture.stat_interval module parameter.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/rcutorture/configs/rcu/TASKS02.boot

 rcutorture.torture_type=tasks
+rcutorture.stat_interval=60

tools/testing/selftests/rcutorture/configs/rcu/ver_functions.sh

@@ -9,7 +9,7 @@
 
 # rcutorture_param_n_barrier_cbs bootparam-string
 #
-# Adds n_barrier_cbs rcutorture module parameter to kernels having it.
+# Adds n_barrier_cbs rcutorture module parameter if not already specified.
 rcutorture_param_n_barrier_cbs () {
 	if echo $1 | grep -q "rcutorture\.n_barrier_cbs"
 	then
@@ -30,13 +30,25 @@ rcutorture_param_onoff () {
 	fi
 }
 
+# rcutorture_param_stat_interval bootparam-string
+#
+# Adds stat_interval rcutorture module parameter if not already specified.
+rcutorture_param_stat_interval () {
+	if echo $1 | grep -q "rcutorture\.stat_interval"
+	then
+		:
+	else
+		echo rcutorture.stat_interval=15
+	fi
+}
+
 # per_version_boot_params bootparam-string config-file seconds
 #
 # Adds per-version torture-module parameters to kernels supporting them.
 per_version_boot_params () {
 	echo $1 `rcutorture_param_onoff "$1" "$2"` \
 		`rcutorture_param_n_barrier_cbs "$1"` \
-		rcutorture.stat_interval=15 \
+		`rcutorture_param_stat_interval "$1"` \
 		rcutorture.shutdown_secs=$3 \
 		rcutorture.test_no_idle_hz=1 \
 		rcutorture.verbose=1