rcutorture: Add support to detect if boost kthread prio is too low

When rcutorture is built in to the kernel, an earlier patch detects
that and raises the priority of RCU's kthreads to allow rcutorture's
RCU priority boosting tests to succeed.

However, if rcutorture is built as a module, those priorities must be
raised manually via the rcutree.kthread_prio kernel boot parameter.
If this manual step is not taken, rcutorture's RCU priority boosting
tests will fail due to kthread starvation.  One approach would be to
raise the default priority, but that risks breaking existing users.
Another approach would be to allow runtime adjustment of RCU's kthread
priorities, but that introduces numerous "interesting" race conditions.
This patch therefore instead detects too-low priorities, and prints a
message and disables the RCU priority boosting tests in that case.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 28 insertions, 4 deletions

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 049b3735dba8..e3d2d4f1d928 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1787,6 +1787,32 @@ static void rcu_torture_barrier_cleanup(void)
 	}
 }
 
+static bool rcu_torture_can_boost(void)
+{
+	static int boost_warn_once;
+	int prio;
+
+	if (!(test_boost == 1 && cur_ops->can_boost) && test_boost != 2)
+		return false;
+
+	prio = rcu_get_gp_kthreads_prio();
+	if (!prio)
+		return false;
+
+	if (prio < 2) {
+		if (boost_warn_once  == 1)
+			return false;
+
+		pr_alert("%s: WARN: RCU kthread priority too low to test boosting. "
+			 "Skipping RCU boost test. Try passing rcutree.kthread_prio > 1 "
+			 "on the kernel command line.\n", KBUILD_MODNAME);
+		boost_warn_once = 1;
+		return false;
+	}
+
+	return true;
+}
+
 static enum cpuhp_state rcutor_hp;
 
 static void
@@ -1831,8 +1857,7 @@ rcu_torture_cleanup(void)
 	torture_stop_kthread(rcu_torture_fqs, fqs_task);
 	for (i = 0; i < ncbflooders; i++)
 		torture_stop_kthread(rcu_torture_cbflood, cbflood_task[i]);
-	if ((test_boost == 1 && cur_ops->can_boost) ||
-	    test_boost == 2)
+	if (rcu_torture_can_boost())
 		cpuhp_remove_state(rcutor_hp);
 
 	/*
@@ -2056,8 +2081,7 @@ rcu_torture_init(void)
 		test_boost_interval = 1;
 	if (test_boost_duration < 2)
 		test_boost_duration = 2;
-	if ((test_boost == 1 && cur_ops->can_boost) ||
-	    test_boost == 2) {
+	if (rcu_torture_can_boost()) {
 
 		boost_starttime = jiffies + test_boost_interval * HZ;