rcu: Don't funnel-lock above leaf node if GP in progress

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@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Add "Startedleaf" tracing as suggested by Joel Fernandes. ]

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 7d3650c..5ab1df0 100644 (file)
--- 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 5643c13..24a79e8 100644 (file)
--- 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)