rcutorture: Disable RT throttling for boost tests

Currently rcutorture is not able to torture RCU boosting properly. This
is because the rcutorture's boost threads which are doing the torturing
may be throttled due to RT throttling.

This patch makes rcutorture use the right torture technique (unthrottled
rcutorture boost tasks) for torturing RCU so that the test fails
correctly when no boost is available.

Currently this requires accessing sysctl_sched_rt_runtime directly, but
that should be Ok since rcutorture is test code. Such direct access is
also only possible if rcutorture is used as a built-in so make it
conditional on that.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/rcutorture.c

@@ -55,6 +55,7 @@
 #include <linux/torture.h>
 #include <linux/vmalloc.h>
 #include <linux/sched/debug.h>
+#include <linux/sched/sysctl.h>
 
 #include "rcu.h"
 
@@ -772,6 +773,32 @@ static void rcu_torture_boost_cb(struct rcu_head *head)
 	smp_store_release(&rbip->inflight, 0);
 }
 
+static int old_rt_runtime = -1;
+
+static void rcu_torture_disable_rt_throttle(void)
+{
+	/*
+	 * Disable RT throttling so that rcutorture's boost threads don't get
+	 * throttled. Only possible if rcutorture is built-in otherwise the
+	 * user should manually do this by setting the sched_rt_period_us and
+	 * sched_rt_runtime sysctls.
+	 */
+	if (!IS_BUILTIN(CONFIG_RCU_TORTURE_TEST) || old_rt_runtime != -1)
+		return;
+
+	old_rt_runtime = sysctl_sched_rt_runtime;
+	sysctl_sched_rt_runtime = -1;
+}
+
+static void rcu_torture_enable_rt_throttle(void)
+{
+	if (!IS_BUILTIN(CONFIG_RCU_TORTURE_TEST) || old_rt_runtime == -1)
+		return;
+
+	sysctl_sched_rt_runtime = old_rt_runtime;
+	old_rt_runtime = -1;
+}
+
 static int rcu_torture_boost(void *arg)
 {
 	unsigned long call_rcu_time;
@@ -1511,6 +1538,7 @@ static int rcutorture_booster_cleanup(unsigned int cpu)
 	mutex_lock(&boost_mutex);
 	t = boost_tasks[cpu];
 	boost_tasks[cpu] = NULL;
+	rcu_torture_enable_rt_throttle();
 	mutex_unlock(&boost_mutex);
 
 	/* This must be outside of the mutex, otherwise deadlock! */
@@ -1527,6 +1555,7 @@ static int rcutorture_booster_init(unsigned int cpu)
 
 	/* Don't allow time recalculation while creating a new task. */
 	mutex_lock(&boost_mutex);
+	rcu_torture_disable_rt_throttle();
 	VERBOSE_TOROUT_STRING("Creating rcu_torture_boost task");
 	boost_tasks[cpu] = kthread_create_on_node(rcu_torture_boost, NULL,
 						  cpu_to_node(cpu),