[PATCH tip/core/rcu 32/40] rcu: Don't funnel-lock above leaf node if GP in progress

[Paul E. McKenney]

The old grace-period start code would acquire only the leaf's rcu_node
structure's ->lock if that structure believed that a grace period was
in progress.  The new code advances to the leaf's parent in this case,
needlessly acquiring then leaf's parent's ->lock.  This commit therefore
checks the grace-period state after marking the leaf with the need for
the specified grace period, and if the leaf believes that a grace period
is in progress, takes an early exit.

Reported-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
[ paulmck: Add "Startedleaf" tracing as suggested by Joel Fernandes. ]
---
 include/trace/events/rcu.h | 4 ++--
 kernel/rcu/tree.c          | 9 +++++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 7d3650cc9d30..5ab1df0a2801 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -91,8 +91,8 @@ TRACE_EVENT(rcu_grace_period,
  *
  * "Startleaf": Request a grace period based on leaf-node data.
  * "Prestarted": Someone beat us to the request
- * "Startedleaf": Leaf-node start proved sufficient.
- * "Startedleafroot": Leaf-node start proved sufficient after checking root.
+ * "Startedleaf": Leaf node marked for future GP.
+ * "Startedleafroot": All nodes from leaf to root marked for future GP.
  * "Startedroot": Requested a nocb grace period based on root-node data.
  * "NoGPkthread": The RCU grace-period kthread has not yet started.
  * "StartWait": Start waiting for the requested grace period.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fe986771c5cf..1c2aa020688a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1590,6 +1590,15 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 			goto unlock_out;
 		}
 		rnp_root->gp_seq_needed = c;
+		if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
+			/*
+			 * We just marked the leaf, and a grace period
+			 * is in progress, which means that rcu_gp_cleanup()
+			 * will see the marking.  Bail to reduce contention.
+			 */
+			trace_rcu_this_gp(rnp, rdp, c, TPS("Startedleaf"));
+			goto unlock_out;
+		}
 		if (rnp_root != rnp && rnp_root->parent != NULL)
 			raw_spin_unlock_rcu_node(rnp_root);
 		if (!rnp_root->parent)
-- 
2.17.1