rcu-tasks: Maintain real-time response in rcu_tasks_postscan()

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.

[ paulmck: Apply Frederic Weisbecker feedback. ]

Link: https://lore.kernel.org/all/ZdeI_-RfdLR8jlsm@localhost.localdomain/
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>

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index c61dc92537dbb085c545dd6c90bcab2ff81ab312..147b5945d67a046dd568e6df586881f8c1a0c25f 100644 (file)
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -968,13 +968,33 @@ 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);
+                       t1 = list_entry(tmp.next, struct task_struct, rcu_tasks_exit_list);
+                       list_del(&tmp);
+                       j = jiffies + 1;
+               }
                raw_spin_unlock_irq_rcu_node(rtpcp);
        }