rcu-tasks: Add IPI failure count to statistics

This commit adds a failure-return count for smp_call_function_single(),
and adds this to the console messages for rcutorture writer stalls and at
the end of rcutorture testing.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -32,6 +32,7 @@ typedef void (*postgp_func_t)(struct rcu_tasks *rtp);
  * @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.
+ * @n_ipis_fails: Number of IPI-send failures.
  * @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).
@@ -51,6 +52,7 @@ struct rcu_tasks {
 	unsigned long gp_start;
 	unsigned long n_gps;
 	unsigned long n_ipis;
+	unsigned long n_ipis_fails;
 	struct task_struct *kthread_ptr;
 	rcu_tasks_gp_func_t gp_func;
 	pregp_func_t pregp_func;
@@ -290,12 +292,12 @@ 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 g:%lu i:%lu %c%c %s\n",
+	pr_info("%s: %s(%d) since %lu g:%lu i:%lu/%lu %c%c %s\n",
 		rtp->kname,
-		tasks_gp_state_getname(rtp),
-		data_race(rtp->gp_state),
+		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),
+		data_race(rtp->n_gps),
+		data_race(rtp->n_ipis_fails), data_race(rtp->n_ipis),
 		".k"[!!data_race(rtp->kthread_ptr)],
 		".C"[!!data_race(rtp->cbs_head)],
 		s);
@@ -909,6 +911,7 @@ static void trc_wait_for_one_reader(struct task_struct *t,
 					     trc_read_check_handler, t, 0)) {
 			// Just in case there is some other reason for
 			// failure than the target CPU being offline.
+			rcu_tasks_trace.n_ipis_fails++;
 			per_cpu(trc_ipi_to_cpu, cpu) = false;
 			t->trc_ipi_to_cpu = cpu;
 			if (atomic_dec_and_test(&trc_n_readers_need_end)) {