rcutorture: Add flag to produce non-busy-wait task stalls

This commit aids testing of RCU task stall warning messages by adding
an rcutorture.stall_cpu_block module parameter that results in the
induced stall sleeping within the RCU read-side critical section.
Spinning with interrupts disabled is still available via the
rcutorture.stall_cpu_irqsoff module parameter, and specifying neither
of these two module parameters will spin with preemption disabled.

Note that sleeping (as opposed to preemption) results in additional
complaints from RCU at context-switch time, so yet more testing.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -4210,6 +4210,11 @@
 			Duration of CPU stall (s) to test RCU CPU stall
 			warnings, zero to disable.
 
+	rcutorture.stall_cpu_block= [KNL]
+			Sleep while stalling if set.  This will result
+			in warnings from preemptible RCU in addition
+			to any other stall-related activity.
+
 	rcutorture.stall_cpu_holdoff= [KNL]
 			Time to wait (s) after boot before inducing stall.
 

kernel/rcu/rcutorture.c

@@ -114,6 +114,7 @@ torture_param(int, stall_cpu, 0, "Stall duration (s), zero to disable.");
 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, stat_interval, 60,
 	     "Number of seconds between stats printk()s");
 torture_param(int, stutter, 5, "Number of seconds to run/halt test");
@@ -1548,6 +1549,7 @@ rcu_torture_print_module_parms(struct rcu_torture_ops *cur_ops, const char *tag)
 		 "test_boost=%d/%d test_boost_interval=%d "
 		 "test_boost_duration=%d shutdown_secs=%d "
 		 "stall_cpu=%d stall_cpu_holdoff=%d stall_cpu_irqsoff=%d "
+		 "stall_cpu_block=%d "
 		 "n_barrier_cbs=%d "
 		 "onoff_interval=%d onoff_holdoff=%d\n",
 		 torture_type, tag, nrealreaders, nfakewriters,
@@ -1556,6 +1558,7 @@ rcu_torture_print_module_parms(struct rcu_torture_ops *cur_ops, const char *tag)
 		 test_boost, cur_ops->can_boost,
 		 test_boost_interval, test_boost_duration, shutdown_secs,
 		 stall_cpu, stall_cpu_holdoff, stall_cpu_irqsoff,
+		 stall_cpu_block,
 		 n_barrier_cbs,
 		 onoff_interval, onoff_holdoff);
 }
@@ -1611,6 +1614,7 @@ static int rcutorture_booster_init(unsigned int cpu)
  */
 static int rcu_torture_stall(void *args)
 {
+	int idx;
 	unsigned long stop_at;
 
 	VERBOSE_TOROUT_STRING("rcu_torture_stall task started");
@@ -1622,21 +1626,22 @@ static int rcu_torture_stall(void *args)
 	if (!kthread_should_stop()) {
 		stop_at = ktime_get_seconds() + stall_cpu;
 		/* RCU CPU stall is expected behavior in following code. */
-		rcu_read_lock();
+		idx = cur_ops->readlock();
 		if (stall_cpu_irqsoff)
 			local_irq_disable();
-		else
+		else if (!stall_cpu_block)
 			preempt_disable();
 		pr_alert("rcu_torture_stall start on CPU %d.\n",
-			 smp_processor_id());
+			 raw_smp_processor_id());
 		while (ULONG_CMP_LT((unsigned long)ktime_get_seconds(),
 				    stop_at))
-			continue;  /* Induce RCU CPU stall warning. */
+			if (stall_cpu_block)
+				schedule_timeout_uninterruptible(HZ);
 		if (stall_cpu_irqsoff)
 			local_irq_enable();
-		else
+		else if (!stall_cpu_block)
 			preempt_enable();
-		rcu_read_unlock();
+		cur_ops->readunlock(idx);
 		pr_alert("rcu_torture_stall end.\n");
 	}
 	torture_shutdown_absorb("rcu_torture_stall");