This commit pushes the grace-period-end checks further down into
rcu_dump_cpu_stacks(), and also uses lockless checks coupled with
finer-grained locking.
The result is that the current leaf rcu_node structure's ->lock is
acquired only if a stack backtrace might be needed from the current CPU,
and is held across only that CPU's backtrace. As a result, if there are
no stalled CPUs associated with a given rcu_node structure, then its
->lock will not be acquired at all. On large systems, it is usually
(though not always) the case that a small number of CPUs are stalling
the current grace period, which means that the ->lock need be acquired
only for a small fraction of the rcu_node structures.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
kernel/rcu/tree_stall.h | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index b530844becf85..8994391b95c76 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -342,20 +342,24 @@ static void rcu_dump_cpu_stacks(unsigned long gp_seq)
struct rcu_node *rnp;
rcu_for_each_leaf_node(rnp) {
- if (gp_seq != data_race(rcu_state.gp_seq)) {
- pr_err("INFO: Stall ended during stack backtracing.\n");
- return;
- }
printk_deferred_enter();
- raw_spin_lock_irqsave_rcu_node(rnp, flags);
- for_each_leaf_node_possible_cpu(rnp, cpu)
+ for_each_leaf_node_possible_cpu(rnp, cpu) {
+ if (gp_seq != data_race(rcu_state.gp_seq)) {
+ printk_deferred_exit();
+ pr_err("INFO: Stall ended during stack backtracing.\n");
+ return;
+ }
+ if (!(data_race(rnp->qsmask) & leaf_node_cpu_bit(rnp, cpu)))
+ continue;
+ raw_spin_lock_irqsave_rcu_node(rnp, flags);
if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
if (cpu_is_offline(cpu))
pr_err("Offline CPU %d blocking current GP.\n", cpu);
else
dump_cpu_task(cpu);
+ raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}
- raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ }
printk_deferred_exit();
}
}
--
2.40.1
