diff mbox series

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

Message ID 20240217012745.3446231-7-boqun.feng@gmail.com
State New
Headers
Series RCU tasks fixes for v6.9 |

Commit Message

Boqun Feng Feb. 17, 2024, 1:27 a.m. UTC 
  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(-)

Comments

Frederic Weisbecker Feb. 22, 2024, 5:48 p.m. UTC | #1 
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
>
Paul E. McKenney Feb. 22, 2024, 8:52 p.m. UTC | #2 
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
> >
Paul E. McKenney Feb. 22, 2024, 10:56 p.m. UTC | #3 
On Thu, Feb 22, 2024 at 12:52:24PM -0800, Paul E. McKenney wrote:
> 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;

OK, OK...  ;-)

			t1 = list_entry(tmp.next, struct task_struct, rcu_tasks_exit_list);

Is there still a better way?

							Thanx, Paul

> > Thanks.
> > 
> > 
> > > +			j = jiffies + 1;
> > > +		}
> > >  		raw_spin_unlock_irq_rcu_node(rtpcp);
> > >  	}
> > >  
> > > -- 
> > > 2.43.0
> > >
Frederic Weisbecker Feb. 23, 2024, 12:17 p.m. UTC | #4 
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);
Frederic Weisbecker Feb. 23, 2024, 3:14 p.m. UTC | #5 
On Fri, Feb 23, 2024 at 01:17:14PM +0100, Frederic Weisbecker wrote:
> 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);

Oh no! The task has to stay in the list for subsequent grace periods! Please
forget that suggestion... Yours looks good!

Thanks.

> +
>  			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);
>
Paul E. McKenney Feb. 24, 2024, 12:23 a.m. UTC | #6 
On Fri, Feb 23, 2024 at 04:14:49PM +0100, Frederic Weisbecker wrote:
> On Fri, Feb 23, 2024 at 01:17:14PM +0100, Frederic Weisbecker wrote:
> > 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);
> 
> Oh no! The task has to stay in the list for subsequent grace periods! Please
> forget that suggestion... Yours looks good!

You had me going for a bit, and I do know that feeling!  ;-)

							Thanx, Paul

> Thanks.
> 
> > +
> >  			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);
> >
diff mbox series

Patch

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);
+			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);
 	}