[PATCH v2 6/6] rcu-tasks: Maintain real-time response in rcu_tasks_postscan()

[Boqun Feng <boqun.feng@gmail.com>]

From: "Paul E. McKenney" <paulmck@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 <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Boqun Feng <boqun.feng@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