[PATCH] Extend RCU torture module to test tickless idle CPU

This patch forces RCU torture threads off various CPUs in the system
allowing them to become idle and go tickless.  Meant to test support for
such tickless idle CPU in RCU.
Signed-off-by: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/rcutorture.c

@@ -48,9 +48,11 @@
 MODULE_LICENSE("GPL");
 
 static int nreaders = -1;	/* # reader threads, defaults to 4*ncpus */
-static int stat_interval = 0;	/* Interval between stats, in seconds. */
+static int stat_interval;	/* Interval between stats, in seconds. */
 				/*  Defaults to "only at end of test". */
-static int verbose = 0;		/* Print more debug info. */
+static int verbose;		/* Print more debug info. */
+static int test_no_idle_hz;	/* Test RCU's support for tickless idle CPUs. */
+static int shuffle_interval = 5; /* Interval between shuffles (in sec)*/
 
 MODULE_PARM(nreaders, "i");
 MODULE_PARM_DESC(nreaders, "Number of RCU reader threads");
@@ -58,6 +60,10 @@ MODULE_PARM(stat_interval, "i");
 MODULE_PARM_DESC(stat_interval, "Number of seconds between stats printk()s");
 MODULE_PARM(verbose, "i");
 MODULE_PARM_DESC(verbose, "Enable verbose debugging printk()s");
+MODULE_PARM(test_no_idle_hz, "i");
+MODULE_PARM_DESC(test_no_idle_hz, "Test support for tickless idle CPUs");
+MODULE_PARM(shuffle_interval, "i");
+MODULE_PARM_DESC(shuffle_interval, "Number of seconds between shuffles");
 #define TORTURE_FLAG "rcutorture: "
 #define PRINTK_STRING(s) \
 	do { printk(KERN_ALERT TORTURE_FLAG s "\n"); } while (0)
@@ -72,6 +78,7 @@ static int nrealreaders;
 static struct task_struct *writer_task;
 static struct task_struct **reader_tasks;
 static struct task_struct *stats_task;
+static struct task_struct *shuffler_task;
 
 #define RCU_TORTURE_PIPE_LEN 10
 
@@ -375,12 +382,77 @@ rcu_torture_stats(void *arg)
 	return 0;
 }
 
+static int rcu_idle_cpu;	/* Force all torture tasks off this CPU */
+
+/* Shuffle tasks such that we allow @rcu_idle_cpu to become idle. A special case
+ * is when @rcu_idle_cpu = -1, when we allow the tasks to run on all CPUs.
+ */
+void rcu_torture_shuffle_tasks(void)
+{
+	cpumask_t tmp_mask = CPU_MASK_ALL;
+	int i;
+
+	lock_cpu_hotplug();
+
+	/* No point in shuffling if there is only one online CPU (ex: UP) */
+	if (num_online_cpus() == 1) {
+		unlock_cpu_hotplug();
+		return;
+	}
+
+	if (rcu_idle_cpu != -1)
+		cpu_clear(rcu_idle_cpu, tmp_mask);
+
+	set_cpus_allowed(current, tmp_mask);
+
+	if (reader_tasks != NULL) {
+		for (i = 0; i < nrealreaders; i++)
+			if (reader_tasks[i])
+				set_cpus_allowed(reader_tasks[i], tmp_mask);
+	}
+
+	if (writer_task)
+		set_cpus_allowed(writer_task, tmp_mask);
+
+	if (stats_task)
+		set_cpus_allowed(stats_task, tmp_mask);
+
+	if (rcu_idle_cpu == -1)
+		rcu_idle_cpu = num_online_cpus() - 1;
+	else
+		rcu_idle_cpu--;
+
+	unlock_cpu_hotplug();
+}
+
+/* Shuffle tasks across CPUs, with the intent of allowing each CPU in the
+ * system to become idle at a time and cut off its timer ticks. This is meant
+ * to test the support for such tickless idle CPU in RCU.
+ */
+static int
+rcu_torture_shuffle(void *arg)
+{
+	VERBOSE_PRINTK_STRING("rcu_torture_shuffle task started");
+	do {
+		schedule_timeout_interruptible(shuffle_interval * HZ);
+		rcu_torture_shuffle_tasks();
+	} while (!kthread_should_stop());
+	VERBOSE_PRINTK_STRING("rcu_torture_shuffle task stopping");
+	return 0;
+}
+
 static void
 rcu_torture_cleanup(void)
 {
 	int i;
 
 	fullstop = 1;
+	if (shuffler_task != NULL) {
+		VERBOSE_PRINTK_STRING("Stopping rcu_torture_shuffle task");
+		kthread_stop(shuffler_task);
+	}
+	shuffler_task = NULL;
+
 	if (writer_task != NULL) {
 		VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task");
 		kthread_stop(writer_task);
@@ -429,9 +501,11 @@ rcu_torture_init(void)
 		nrealreaders = nreaders;
 	else
 		nrealreaders = 2 * num_online_cpus();
-	printk(KERN_ALERT TORTURE_FLAG
-	       "--- Start of test: nreaders=%d stat_interval=%d verbose=%d\n",
-	       nrealreaders, stat_interval, verbose);
+	printk(KERN_ALERT TORTURE_FLAG "--- Start of test: nreaders=%d "
+		"stat_interval=%d verbose=%d test_no_idle_hz=%d "
+		"shuffle_interval = %d\n",
+		nrealreaders, stat_interval, verbose, test_no_idle_hz,
+		shuffle_interval);
 	fullstop = 0;
 
 	/* Set up the freelist. */
@@ -501,6 +575,18 @@ rcu_torture_init(void)
 			goto unwind;
 		}
 	}
+	if (test_no_idle_hz) {
+		rcu_idle_cpu = num_online_cpus() - 1;
+		/* Create the shuffler thread */
+		shuffler_task = kthread_run(rcu_torture_shuffle, NULL,
+					  "rcu_torture_shuffle");
+		if (IS_ERR(shuffler_task)) {
+			firsterr = PTR_ERR(shuffler_task);
+			VERBOSE_PRINTK_ERRSTRING("Failed to create shuffler");
+			shuffler_task = NULL;
+			goto unwind;
+		}
+	}
 	return 0;
 
 unwind: