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

[Paul E. McKenney]

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@xxxxxxxxxx>
> > 
> > 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@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Sebastian Siewior <bigeasy@xxxxxxxxxxxxx>
> > Cc: Anna-Maria Behnsen <anna-maria@xxxxxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > ---
> >  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
> >