From: "Paul E. McKenney" <paul...@kernel.org>
The current code will scan the entirety of each per-CPU list of exiting
tasks in ->rtp_exit_list with interrupts disabled. This is normally just
fine, because each CPU typically won't have very many tasks in this state.
However, if a large number of tasks block late in do_exit(), these lists
could be arbitrarily long. Low probability, perhaps, but it really
could happen.
This commit therefore occasionally re-enables interrupts while traversing
these lists, inserting a dummy element to hold the current place in the
list. In kernels built with CONFIG_PREEMPT_RT=y, this re-enabling happens
after each list element is processed, otherwise every one-to-two jiffies.
Signed-off-by: Paul E. McKenney <paul...@kernel.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Sebastian Siewior <bige...@linutronix.de>
Cc: Anna-Maria Behnsen <anna-ma...@linutronix.de>
Cc: Steven Rostedt <rost...@goodmis.org>
Signed-off-by: Boqun Feng <boqun.f...@gmail.com>
---
kernel/rcu/tasks.h | 21 ++++++++++++++++++++-
1 file changed, 20 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 4dc355b2ac22..866743e0796f 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -971,13 +971,32 @@ static void rcu_tasks_postscan(struct list_head *hop)
*/
for_each_possible_cpu(cpu) {
+ unsigned long j = jiffies + 1;
struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu,
cpu);
struct task_struct *t;
+ struct task_struct *t1;
+ struct list_head tmp;
raw_spin_lock_irq_rcu_node(rtpcp);
- list_for_each_entry(t, &rtpcp->rtp_exit_list,
rcu_tasks_exit_list)
+ list_for_each_entry_safe(t, t1, &rtpcp->rtp_exit_list,
rcu_tasks_exit_list) {
if (list_empty(&t->rcu_tasks_holdout_list))
rcu_tasks_pertask(t, hop);
+
+ // RT kernels need frequent pauses, otherwise
+ // pause at least once per pair of jiffies.
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
time_before(jiffies, j))
+ continue;
+
+ // Keep our place in the list while pausing.
+ // Nothing else traverses this list, so adding a
+ // bare list_head is OK.
+ list_add(&tmp, &t->rcu_tasks_exit_list);
+ raw_spin_unlock_irq_rcu_node(rtpcp);
+ cond_resched(); // For CONFIG_PREEMPT=n kernels
+ raw_spin_lock_irq_rcu_node(rtpcp);
+ list_del(&tmp);
+ j = jiffies + 1;
+ }
raw_spin_unlock_irq_rcu_node(rtpcp);
}
--
2.43.0
