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

Paul E. McKenney
2024-02-22T20:52:25Z

On Thu, Feb 22, 2024 at 06:48:47PM +0100, Frederic Weisbecker wrote:
> 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.

That is OK, courtesy of the _safe in list_for_each_entry_safe().

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

It can, and that is a problem, good catch!

My current thought is to add this before the list_del(), which is
admittedly a bit crude:

t1 = tmp.next;

Is there a better way?

Thanx, Paul

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