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. ]
*
* "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.
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)