rcuscale: Measure RCU Tasks Trace grace-period kthread CPU time

This commit causes RCU Tasks Trace to output the CPU time consumed by
its grace-period kthread.  The CPU time is whatever is in the designated
task's current->stime field, and thus is controlled by whatever CPU-time
accounting scheme is in effect.

This output appears in microseconds as follows on the console:

rcu_scale: Grace-period kthread CPU time: 42367.037

[ paulmck: Apply Willy Tarreau feedback. ]

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

Author: paulmckrcu

kernel/rcu/rcu.h

@@ -505,6 +505,9 @@ void rcu_async_relax(void);
 void rcupdate_announce_bootup_oddness(void);
 #ifdef CONFIG_TASKS_RCU_GENERIC
 void show_rcu_tasks_gp_kthreads(void);
+# ifdef CONFIG_TASKS_RCU
+struct task_struct *get_rcu_tasks_gp_kthread(void);
+# endif // # ifdef CONFIG_TASKS_RCU
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
 static inline void show_rcu_tasks_gp_kthreads(void) {}
 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */

kernel/rcu/rcuscale.c

@@ -298,6 +298,7 @@ static struct rcu_scale_ops tasks_ops = {
 	.gp_barrier	= rcu_barrier_tasks,
 	.sync		= synchronize_rcu_tasks,
 	.exp_sync	= synchronize_rcu_tasks,
+	.rso_gp_kthread	= get_rcu_tasks_gp_kthread,
 	.name		= "tasks"
 };
 

kernel/rcu/tasks.h

@@ -1042,6 +1042,12 @@ void show_rcu_tasks_classic_gp_kthread(void)
 EXPORT_SYMBOL_GPL(show_rcu_tasks_classic_gp_kthread);
 #endif // !defined(CONFIG_TINY_RCU)
 
+struct task_struct *get_rcu_tasks_gp_kthread(void)
+{
+	return rcu_tasks.kthread_ptr;
+}
+EXPORT_SYMBOL_GPL(get_rcu_tasks_gp_kthread);
+
 /*
  * Contribute to protect against tasklist scan blind spot while the
  * task is exiting and may be removed from the tasklist. See