rcu: Allow rcutorture to starve grace-period kthread

This commit provides an rcutorture.stall_gp_kthread module parameter
to allow rcutorture to starve the grace-period kthread.  This allows
testing the code that detects such starvation.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -4221,6 +4221,13 @@
 	rcutorture.stall_cpu_irqsoff= [KNL]
 			Disable interrupts while stalling if set.
 
+	rcutorture.stall_gp_kthread= [KNL]
+			Duration (s) of forced sleep within RCU
+			grace-period kthread to test RCU CPU stall
+			warnings, zero to disable.  If both stall_cpu
+			and stall_gp_kthread are specified, the
+			kthread is starved first, then the CPU.
+
 	rcutorture.stat_interval= [KNL]
 			Time (s) between statistics printk()s.
 

kernel/rcu/rcu.h

@@ -454,6 +454,7 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
 			       unsigned long secs,
 			       unsigned long c_old,
 			       unsigned long c);
+void rcu_gp_set_torture_wait(int duration);
 #else
 static inline void rcutorture_get_gp_data(enum rcutorture_type test_type,
 					  int *flags, unsigned long *gp_seq)
@@ -471,6 +472,7 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
 #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \
 	do { } while (0)
 #endif
+static inline void rcu_gp_set_torture_wait(int duration) { }
 #endif
 
 #if IS_ENABLED(CONFIG_RCU_TORTURE_TEST) || IS_MODULE(CONFIG_RCU_TORTURE_TEST)

kernel/rcu/rcutorture.c

@@ -115,6 +115,8 @@ torture_param(int, stall_cpu_holdoff, 10,
 	     "Time to wait before starting stall (s).");
 torture_param(int, stall_cpu_irqsoff, 0, "Disable interrupts while stalling.");
 torture_param(int, stall_cpu_block, 0, "Sleep while stalling.");
+torture_param(int, stall_gp_kthread, 0,
+	      "Grace-period kthread stall duration (s).");
 torture_param(int, stat_interval, 60,
 	     "Number of seconds between stats printk()s");
 torture_param(int, stutter, 5, "Number of seconds to run/halt test");
@@ -1623,7 +1625,17 @@ static int rcu_torture_stall(void *args)
 		schedule_timeout_interruptible(stall_cpu_holdoff * HZ);
 		VERBOSE_TOROUT_STRING("rcu_torture_stall end holdoff");
 	}
-	if (!kthread_should_stop()) {
+	if (!kthread_should_stop() && stall_gp_kthread > 0) {
+		VERBOSE_TOROUT_STRING("rcu_torture_stall begin GP stall");
+		rcu_gp_set_torture_wait(stall_gp_kthread * HZ);
+		for (idx = 0; idx < stall_gp_kthread + 2; idx++) {
+			if (kthread_should_stop())
+				break;
+			schedule_timeout_uninterruptible(HZ);
+		}
+	}
+	if (!kthread_should_stop() && stall_cpu > 0) {
+		VERBOSE_TOROUT_STRING("rcu_torture_stall begin CPU stall");
 		stop_at = ktime_get_seconds() + stall_cpu;
 		/* RCU CPU stall is expected behavior in following code. */
 		idx = cur_ops->readlock();
@@ -1642,8 +1654,8 @@ static int rcu_torture_stall(void *args)
 		else if (!stall_cpu_block)
 			preempt_enable();
 		cur_ops->readunlock(idx);
-		pr_alert("rcu_torture_stall end.\n");
 	}
+	pr_alert("rcu_torture_stall end.\n");
 	torture_shutdown_absorb("rcu_torture_stall");
 	while (!kthread_should_stop())
 		schedule_timeout_interruptible(10 * HZ);
@@ -1653,7 +1665,7 @@ static int rcu_torture_stall(void *args)
 /* Spawn CPU-stall kthread, if stall_cpu specified. */
 static int __init rcu_torture_stall_init(void)
 {
-	if (stall_cpu <= 0)
+	if (stall_cpu <= 0 && stall_gp_kthread <= 0)
 		return 0;
 	return torture_create_kthread(rcu_torture_stall, NULL, stall_task);
 }

kernel/rcu/tree.c

@@ -1486,6 +1486,31 @@ static void rcu_gp_slow(int delay)
 		schedule_timeout_uninterruptible(delay);
 }
 
+static unsigned long sleep_duration;
+
+/* Allow rcutorture to stall the grace-period kthread. */
+void rcu_gp_set_torture_wait(int duration)
+{
+	if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST) && duration > 0)
+		WRITE_ONCE(sleep_duration, duration);
+}
+EXPORT_SYMBOL_GPL(rcu_gp_set_torture_wait);
+
+/* Actually implement the aforementioned wait. */
+static void rcu_gp_torture_wait(void)
+{
+	unsigned long duration;
+
+	if (!IS_ENABLED(CONFIG_RCU_TORTURE_TEST))
+		return;
+	duration = xchg(&sleep_duration, 0UL);
+	if (duration > 0) {
+		pr_alert("%s: Waiting %lu jiffies\n", __func__, duration);
+		schedule_timeout_uninterruptible(duration);
+		pr_alert("%s: Wait complete\n", __func__);
+	}
+}
+
 /*
  * Initialize a new grace period.  Return false if no grace period required.
  */
@@ -1686,6 +1711,7 @@ static void rcu_gp_fqs_loop(void)
 		rcu_state.gp_state = RCU_GP_WAIT_FQS;
 		ret = swait_event_idle_timeout_exclusive(
 				rcu_state.gp_wq, rcu_gp_fqs_check_wake(&gf), j);
+		rcu_gp_torture_wait();
 		rcu_state.gp_state = RCU_GP_DOING_FQS;
 		/* Locking provides needed memory barriers. */
 		/* If grace period done, leave loop. */
@@ -1834,6 +1860,7 @@ static int __noreturn rcu_gp_kthread(void *unused)
 			swait_event_idle_exclusive(rcu_state.gp_wq,
 					 READ_ONCE(rcu_state.gp_flags) &
 					 RCU_GP_FLAG_INIT);
+			rcu_gp_torture_wait();
 			rcu_state.gp_state = RCU_GP_DONE_GPS;
 			/* Locking provides needed memory barrier. */
 			if (rcu_gp_init())