sched: update __cond_resched comment about RCU quiescent states

Update comment in __cond_resched() clarifying how urgently needed
quiescent state are provided.

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

kernel/sched/core.c

@@ -7289,7 +7289,7 @@ int __sched __cond_resched(void)
 		return 1;
 	}
 	/*
-	 * In preemptible kernels, ->rcu_read_lock_nesting tells the tick
+	 * In PREEMPT_RCU kernels, ->rcu_read_lock_nesting tells the tick
 	 * whether the current CPU is in an RCU read-side critical section,
 	 * so the tick can report quiescent states even for CPUs looping
 	 * in kernel context.  In contrast, in non-preemptible kernels,
@@ -7298,6 +7298,8 @@ int __sched __cond_resched(void)
 	 * RCU quiescent state.  Therefore, the following code causes
 	 * cond_resched() to report a quiescent state, but only when RCU
 	 * is in urgent need of one.
+	 * A third case, preemptible, but non-PREEMPT_RCU provides for
+	 * urgently needed quiescent states via rcu_flavor_sched_clock_irq().
 	 */
 #ifndef CONFIG_PREEMPT_RCU
 	rcu_all_qs();