rcu/nocb: Use separate flag to indicate offloaded ->cblist

RCU callback processing currently uses rcu_is_nocb_cpu() to determine
whether or not the current CPU's callbacks are to be offloaded.
This works, but it is not so good for cache locality.  Plus use of
->cblist for offloaded callbacks will greatly increase the frequency
of these checks.  This commit therefore adds a ->offloaded flag to the
rcu_segcblist structure to provide a more flexible and cache-friendly
means of checking for callback offloading.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/rcu_segcblist.c

@@ -73,6 +73,18 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
 	rsclp->enabled = 0;
 }
 
+/*
+ * Mark the specified rcu_segcblist structure as offloaded.  This
+ * structure must be empty.
+ */
+void rcu_segcblist_offload(struct rcu_segcblist *rsclp)
+{
+	WARN_ON_ONCE(!rcu_segcblist_empty(rsclp));
+	WARN_ON_ONCE(rcu_segcblist_n_cbs(rsclp));
+	WARN_ON_ONCE(rcu_segcblist_n_lazy_cbs(rsclp));
+	rsclp->offloaded = 1;
+}
+
 /*
  * Does the specified rcu_segcblist structure contain callbacks that
  * are ready to be invoked?