rcu-tasks: Add a grace-period start time for throttling and debug

This commit adds a place to record the grace-period start in jiffies.
This will be used by later commits for debugging purposes and to throttle
IPIs early in the grace period.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -46,6 +46,7 @@ struct rcu_tasks {
 	raw_spinlock_t cbs_lock;
 	int gp_state;
 	unsigned long gp_jiffies;
+	unsigned long gp_start;
 	struct task_struct *kthread_ptr;
 	rcu_tasks_gp_func_t gp_func;
 	pregp_func_t pregp_func;
@@ -200,6 +201,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 
 		// Wait for one grace period.
 		set_tasks_gp_state(rtp, RTGS_WAIT_GP);
+		rtp->gp_start = jiffies;
 		rtp->gp_func(rtp);
 
 		/* Invoke the callbacks. */