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

Frederic Weisbecker
2024-02-22T17:48:49Z

Le Fri, Feb 16, 2024 at 05:27:41PM -0800, Boqun Feng a écrit :
> 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);

I'm a bit confused about what this does...

> + raw_spin_unlock_irq_rcu_node(rtpcp);
> + cond_resched(); // For CONFIG_PREEMPT=n kernels
> + raw_spin_lock_irq_rcu_node(rtpcp);
> + list_del(&tmp);

Isn't there a risk that t is reaped by then? If it was not observed on_rq
while calling rcu_tasks_pertask() then there is no get_task_struct.

And what about t1? Can't it be reaped as well?

Thanks.

> + j = jiffies + 1;
> + }
> raw_spin_unlock_irq_rcu_node(rtpcp);
> }
>
> --
> 2.43.0
>