[PATCH tip/core/rcu 13/20] rcu: Don't wake rcuc/X kthreads on NOCB CPUs

[Paul E. McKenney]

Chris Friesen notice that rcuc/X kthreads were consuming CPU even on
NOCB CPUs.  This makes no sense because the only purpose or these
kthreads is to invoke normal (non-offloaded) callbacks, of which there
will never be any on NOCB CPUs.  This problem was due to a bug in
cpu_has_callbacks_ready_to_invoke(), which should have been checking
->nxttail[RCU_NEXT_TAIL] for NULL, but which was instead (incorrectly)
checking ->nxttail[RCU_DONE_TAIL].  Because ->nxttail[RCU_DONE_TAIL] is
never NULL, the only effect is to cause the rcuc/X kthread to execute
when it should not do so.

This commit therefore checks ->nxttail[RCU_NEXT_TAIL], which is NULL
for NOCB CPUs.

Reported-by: Chris Friesen <chris.friesen@xxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ed5a17aca281..cac80e08b353 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -611,7 +611,7 @@ static int
 cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
 {
 	return &rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL] &&
-	       rdp->nxttail[RCU_DONE_TAIL] != NULL;
+	       rdp->nxttail[RCU_NEXT_TAIL] != NULL;
 }
 
 /*
-- 
2.5.2