rcu: Prevent early boot set_need_resched() from __rcu_pending()

There isn't a whole lot of point in poking the scheduler before there
are other tasks to switch to.  This commit therefore adds a check
for rcu_scheduler_fully_active in __rcu_pending() to suppress any
pre-scheduler calls to set_need_resched().  The downside of this approach
is additional runtime overhead in a reasonably hot code path.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index a07bf553e02a06b184a784bfcd4a69d0801bbd63..0051dbf6958eb27b29c0ed95db2e7b7a80cf1ed1 100644 (file)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1707,7 +1707,8 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
        check_cpu_stall(rsp, rdp);
 
        /* Is the RCU core waiting for a quiescent state from this CPU? */
-       if (rdp->qs_pending && !rdp->passed_quiesce) {
+       if (rcu_scheduler_fully_active &&
+           rdp->qs_pending && !rdp->passed_quiesce) {
 
                /*
                 * If force_quiescent_state() coming soon and this CPU