rcu-tasks: Add count for idle tasks on offline CPUs

This commit adds a counter for the number of times the quiescent state
was an idle task associated with an offline CPU, and prints this count
at the end of rcutorture runs and at stall time.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -729,6 +729,7 @@ static DEFINE_PER_CPU(bool, trc_ipi_to_cpu);
 // heavyweight readers executing explicit memory barriers.
 unsigned long n_heavy_reader_attempts;
 unsigned long n_heavy_reader_updates;
+unsigned long n_heavy_reader_ofl_updates;
 
 void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
 DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
@@ -840,6 +841,8 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
 		    !rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
 			return false; // No quiescent state, do it the hard way.
 		n_heavy_reader_updates++;
+		if (ofl)
+			n_heavy_reader_ofl_updates++;
 		in_qs = true;
 	} else {
 		in_qs = likely(!t->trc_reader_nesting);
@@ -1156,7 +1159,8 @@ static void show_rcu_tasks_trace_gp_kthread(void)
 {
 	char buf[64];
 
-	sprintf(buf, "N%d h:%lu/%lu", atomic_read(&trc_n_readers_need_end),
+	sprintf(buf, "N%d h:%lu/%lu/%lu", atomic_read(&trc_n_readers_need_end),
+		data_race(n_heavy_reader_ofl_updates),
 		data_race(n_heavy_reader_updates),
 		data_race(n_heavy_reader_attempts));
 	show_rcu_tasks_generic_gp_kthread(&rcu_tasks_trace, buf);