rcu-tasks: Add grace-period and IPI counts to statistics

This commit adds a grace-period count and a count of IPIs sent since
boot, which is printed in response to rcutorture writer stalls and at
the end of rcutorture testing.  These counts will be used to evaluate
various schemes to reduce the number of IPIs sent.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -30,6 +30,8 @@ typedef void (*postgp_func_t)(struct rcu_tasks *rtp);
  * @gp_state: Grace period's most recent state transition (debugging).
  * @gp_jiffies: Time of last @gp_state transition.
  * @gp_start: Most recent grace-period start in jiffies.
+ * @n_gps: Number of grace periods completed since boot.
+ * @n_ipis: Number of IPIs sent to encourage grace periods to end.
  * @pregp_func: This flavor's pre-grace-period function (optional).
  * @pertask_func: This flavor's per-task scan function (optional).
  * @postscan_func: This flavor's post-task scan function (optional).
@@ -47,6 +49,8 @@ struct rcu_tasks {
 	int gp_state;
 	unsigned long gp_jiffies;
 	unsigned long gp_start;
+	unsigned long n_gps;
+	unsigned long n_ipis;
 	struct task_struct *kthread_ptr;
 	rcu_tasks_gp_func_t gp_func;
 	pregp_func_t pregp_func;
@@ -208,6 +212,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 		set_tasks_gp_state(rtp, RTGS_WAIT_GP);
 		rtp->gp_start = jiffies;
 		rtp->gp_func(rtp);
+		rtp->n_gps++;
 
 		/* Invoke the callbacks. */
 		set_tasks_gp_state(rtp, RTGS_INVOKE_CBS);
@@ -285,11 +290,12 @@ static void __init rcu_tasks_bootup_oddness(void)
 /* Dump out rcutorture-relevant state common to all RCU-tasks flavors. */
 static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
 {
-	pr_info("%s: %s(%d) since %lu %c%c %s\n",
+	pr_info("%s: %s(%d) since %lu g:%lu i:%lu %c%c %s\n",
 		rtp->kname,
 		tasks_gp_state_getname(rtp),
 		data_race(rtp->gp_state),
 		jiffies - data_race(rtp->gp_jiffies),
+		data_race(rtp->n_gps), data_race(rtp->n_ipis),
 		".k"[!!data_race(rtp->kthread_ptr)],
 		".C"[!!data_race(rtp->cbs_head)],
 		s);
@@ -592,6 +598,7 @@ static void rcu_tasks_be_rude(struct work_struct *work)
 // Wait for one rude RCU-tasks grace period.
 static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
 {
+	rtp->n_ipis += cpumask_weight(cpu_online_mask);
 	schedule_on_each_cpu(rcu_tasks_be_rude);
 }
 
@@ -856,6 +863,7 @@ static void trc_wait_for_one_reader(struct task_struct *t,
 		atomic_inc(&trc_n_readers_need_end);
 		per_cpu(trc_ipi_to_cpu, cpu) = true;
 		t->trc_ipi_to_cpu = cpu;
+		rcu_tasks_trace.n_ipis++;
 		if (smp_call_function_single(cpu,
 					     trc_read_check_handler, t, 0)) {
 			// Just in case there is some other reason for