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

[Frederic Weisbecker <frederic@kernel.org>]

On Thu, Feb 22, 2024 at 02:56:46PM -0800, Paul E. McKenney wrote:
> On Thu, Feb 22, 2024 at 12:52:24PM -0800, Paul E. McKenney wrote:
> > > > 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...

Oh, ok now I see what you're doing! My fear was that t goes off but doesn't
remove itself and then the list_del() crashes. But no actually tmp places itself
after t and then if t exits, it removes itself before tmp and that's fine.

> > > 
> > > > +			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;
> 
> OK, OK...  ;-)
> 
> 			t1 = list_entry(tmp.next, struct task_struct, rcu_tasks_exit_list);
> 
> Is there still a better way?

That should work.

An (untested) alternative that fiddles a bit less with list internals could look like this:

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 866743e0796f..0ff2b554f5b5 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -973,12 +973,13 @@ 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_safe(t, t1, &rtpcp->rtp_exit_list, rcu_tasks_exit_list) {
+		while (!list_empty(&rtpcp->rtp_exit_list)) {
+			struct task_struct *t;
+			t = list_first_entry(&rtpcp->rtp_exit_list, typeof(*t), rcu_tasks_exit_list);
+			list_del_init(&t->rcu_tasks_exit_list);
+
 			if (list_empty(&t->rcu_tasks_holdout_list))
 				rcu_tasks_pertask(t, hop);
 
@@ -987,14 +988,9 @@ static void rcu_tasks_postscan(struct list_head *hop)
 			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);
-			list_del(&tmp);
 			j = jiffies + 1;
 		}
 		raw_spin_unlock_irq_rcu_node(rtpcp);
@@ -1219,7 +1215,6 @@ void exit_tasks_rcu_stop(void)
 	struct rcu_tasks_percpu *rtpcp;
 	struct task_struct *t = current;
 
-	WARN_ON_ONCE(list_empty(&t->rcu_tasks_exit_list));
 	rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, t->rcu_tasks_exit_cpu);
 	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
 	list_del_init(&t->rcu_tasks_exit_list);