rcu: Make kernel-mode nohz_full CPUs invoke the RCU core processing

If a nohz_full CPU is idle or executing in userspace, it makes good sense
to keep it out of RCU core processing.  After all, the RCU grace-period
kthread can see its quiescent states and all of its callbacks are
offloaded, so there is nothing for RCU core processing to do.

However, if a nohz_full CPU is executing in kernel space, the RCU
grace-period kthread cannot do anything for it, so such a CPU must report
its own quiescent states.  This commit therefore makes nohz_full CPUs
skip RCU core processing only if the scheduler-clock interrupt caught
them in idle or in userspace.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -497,7 +497,7 @@ module_param_cb(jiffies_till_next_fqs, &next_fqs_jiffies_ops, &jiffies_till_next
 module_param(rcu_kick_kthreads, bool, 0644);
 
 static void force_qs_rnp(int (*f)(struct rcu_data *rdp));
-static int rcu_pending(void);
+static int rcu_pending(int user);
 
 /*
  * Return the number of RCU GPs completed thus far for debug & stats.
@@ -2267,7 +2267,7 @@ void rcu_sched_clock_irq(int user)
 		__this_cpu_write(rcu_data.rcu_urgent_qs, false);
 	}
 	rcu_flavor_sched_clock_irq(user);
-	if (rcu_pending())
+	if (rcu_pending(user))
 		invoke_rcu_core();
 
 	trace_rcu_utilization(TPS("End scheduler-tick"));
@@ -2816,7 +2816,7 @@ EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
  * CPU-local state are performed first.  However, we must check for CPU
  * stalls first, else we might not get a chance.
  */
-static int rcu_pending(void)
+static int rcu_pending(int user)
 {
 	bool gp_in_progress;
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
@@ -2829,8 +2829,8 @@ static int rcu_pending(void)
 	if (rcu_nocb_need_deferred_wakeup(rdp))
 		return 1;
 
-	/* Is this CPU a NO_HZ_FULL CPU that should ignore RCU? */
-	if (rcu_nohz_full_cpu())
+	/* Is this a nohz_full CPU in userspace or idle?  (Ignore RCU if so.) */
+	if ((user || rcu_is_cpu_rrupt_from_idle()) && rcu_nohz_full_cpu())
 		return 0;
 
 	/* Is the RCU core waiting for a quiescent state from this CPU? */