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