[PATCH tip/core/rcu 10/10] rcu: Don't initiate RCU priority boosting on root rcu_node

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

Because there is no longer any preempted tasks on the root rcu_node, and
because there is no longer ever an rcub kthread for the root rcu_node,
this commit drops the code in force_qs_rnp() that attempts to awaken
the non-existent root rcub kthread.  This is strictly a performance
enhancement, removing a root rcu_node ->lock acquisition and release
along with some tests in rcu_initiate_boost(), ending with the test that
notes that there is no rcub kthread.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3c23e921b438..d574f65ac023 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2481,12 +2481,6 @@ static void force_qs_rnp(struct rcu_state *rsp,
 		}
 		raw_spin_unlock_irqrestore(&rnp->lock, flags);
 	}
-	rnp = rcu_get_root(rsp);
-	if (rnp->qsmask == 0) {
-		raw_spin_lock_irqsave(&rnp->lock, flags);
-		smp_mb__after_unlock_lock();
-		rcu_initiate_boost(rnp, flags); /* releases rnp->lock. */
-	}
 }
 
 /*
-- 
1.8.1.5