[PATCH] RCU: introduce rcu_needs_cpu() interface

With "Paul E. McKenney" <paulmck@us.ibm.com>

Introduce rcu_needs_cpu() interface.  This can be used to tell if there
will be a new rcu batch on a cpu soon by looking at the curlist pointer.
This can be used to avoid to enter a tickless idle state where the cpu
would miss that a new batch is ready when rcu_start_batch would be called
on a different cpu.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/rcupdate.h

@@ -132,6 +132,7 @@ static inline void rcu_bh_qsctr_inc(int cpu)
 }
 
 extern int rcu_pending(int cpu);
+extern int rcu_needs_cpu(int cpu);
 
 /**
  * rcu_read_lock - mark the beginning of an RCU read-side critical section.

kernel/rcupdate.c

@@ -479,12 +479,31 @@ static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
 	return 0;
 }
 
+/*
+ * Check to see if there is any immediate RCU-related work to be done
+ * by the current CPU, returning 1 if so.  This function is part of the
+ * RCU implementation; it is -not- an exported member of the RCU API.
+ */
 int rcu_pending(int cpu)
 {
 	return __rcu_pending(&rcu_ctrlblk, &per_cpu(rcu_data, cpu)) ||
 		__rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu));
 }
 
+/*
+ * Check to see if any future RCU-related work will need to be done
+ * by the current CPU, even if none need be done immediately, returning
+ * 1 if so.  This function is part of the RCU implementation; it is -not-
+ * an exported member of the RCU API.
+ */
+int rcu_needs_cpu(int cpu)
+{
+	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
+	struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu);
+
+	return (!!rdp->curlist || !!rdp_bh->curlist || rcu_pending(cpu));
+}
+
 void rcu_check_callbacks(int cpu, int user)
 {
 	if (user ||