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 <[email protected]> --- kernel/rcu/tree.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 78dd96e8ce3e..324ca1b2b233 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); -- 2.17.1
