Commit-ID: 928164351e700f91ab588f20fe470cac9db477a9
Gitweb: https://git.kernel.org/tip/928164351e700f91ab588f20fe470cac9db477a9
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
AuthorDate: Wed, 2 May 2018 11:07:02 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Thu, 12 Jul 2018 15:39:01 -0700
rcu: Prevent useless FQS scan after all CPUs have checked in
The force_qs_rnp() function checks for ->qsmask being all zero, that is,
all CPUs for the current rcu_node structure having already passed through
quiescent states. But with RCU-preempt, this is not sufficient to report
quiescent states further up the tree, so there are further checks that
can initiate RCU priority boosting and also for races with CPU-hotplug
operations. However, if neither of these further checks apply, the code
proceeds to carry out a useless scan of an all-zero ->qsmask.
This commit therefore adds code to release the current rcu_node
structure's lock and continue on to the next rcu_node structure, thereby
avoiding this useless scan.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
kernel/rcu/tree.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 439228b79811..3efd591fcd15 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2672,6 +2672,8 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *rsp))
/* rcu_report_unblock_qs_rnp() rlses ->lock */
continue;
}
+ raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ continue;
}
for_each_leaf_node_possible_cpu(rnp, cpu) {
unsigned long bit = leaf_node_cpu_bit(rnp, cpu);
--
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