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

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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>
This commit is contained in:
Paul E. McKenney 2024-02-01 06:10:26 -08:00 committed by Boqun Feng
parent 1612160b91
commit 0bb11a372f
1 changed files with 21 additions and 1 deletions
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22
kernel/rcu/tasks.h
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@ -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);
}