Date: Tue, 27 Oct 2015 07:31:53 -0500
From: Josh Cartwright <joshc@ni.com>
Subject: net: Make synchronize_rcu_expedited() conditional on !RT_FULL

While the use of synchronize_rcu_expedited() might make
synchronize_net() "faster", it does so at significant cost on RT
systems, as expediting a grace period forcibly preempts any
high-priority RT tasks (via the stop_machine() mechanism).

Without this change, we can observe a latency spike up to 30us with
cyclictest by rapidly unplugging/reestablishing an ethernet link.

Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Josh Cartwright <joshc@ni.com>
Cc: bigeasy@linutronix.de
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20151027123153.GG8245@jcartwri.amer.corp.natinst.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 net/core/dev.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -7538,7 +7538,7 @@ EXPORT_SYMBOL(free_netdev);
 void synchronize_net(void)
 {
 	might_sleep();
-	if (rtnl_is_locked())
+	if (rtnl_is_locked() && !IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
 		synchronize_rcu_expedited();
 	else
 		synchronize_rcu();
