[PATCH tip/core/rcu 01/21] rcu: Improve non-root rcu_cbs_completed() accuracy

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

When rcu_cbs_completed() is invoked on a non-root rcu_node structure,
it unconditionally assumes that two grace periods must complete before
the callbacks at hand can be invoked.  This is overly conservative because
if that non-root rcu_node structure believes that no grace period is in
progress, and if the corresponding rcu_state structure's ->gpnum field
has not yet been incremented, then these callbacks may safely be invoked
after only one grace period has completed.

This change is required to permit grace-period start requests to use
funnel locking, which is in turn permitted to reduce root rcu_node ->lock
contention, which has been observed by Nick Piggin.  Furthermore, such
contention will likely be increased by the merging of RCU-bh, RCU-preempt,
and RCU-sched, so it makes sense to take steps to decrease it.

This commit therefore improves the accuracy of rcu_cbs_completed() when
invoked on a non-root rcu_node structure as described above.

Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2a734692a581..f5ca72f2ed43 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1642,6 +1642,21 @@ static unsigned long rcu_cbs_completed(struct rcu_state *rsp,
 		return rnp->completed + 1;
 
 	/*
+	 * If the current rcu_node structure believes that RCU is
+	 * idle, and if the rcu_state structure does not yet reflect
+	 * the start of a new grace period, then the next grace period
+	 * will suffice.  The memory barrier is needed to accurately
+	 * sample the rsp->gpnum, and pairs with the second lock
+	 * acquisition in rcu_gp_init(), which is augmented with
+	 * smp_mb__after_unlock_lock() for this purpose.
+	 */
+	if (rnp->gpnum == rnp->completed) {
+		smp_mb(); /* See above block comment. */
+		if (READ_ONCE(rsp->gpnum) == rnp->completed)
+			return rnp->completed + 1;
+	}
+
+	/*
 	 * Otherwise, wait for a possible partial grace period and
 	 * then the subsequent full grace period.
 	 */
-- 
2.5.2