sched/rt: Make rt_rq->pushable_tasks updates drive rto_mask

  Sebastian noted that the rto_push_work IRQ work can be queued for a CPU
that has an empty pushable_tasks list, which means nothing useful will be
done in the IPI other than queue the work for the next CPU on the rto_mask.

rto_push_irq_work_func() only operates on tasks in the pushable_tasks list,
but the conditions for that irq_work to be queued (and for a CPU to be
added to the rto_mask) rely on rq_rt->nr_migratory instead.

nr_migratory is increased whenever an RT task entity is enqueued and it has
nr_cpus_allowed > 1. Unlike the pushable_tasks list, nr_migratory includes a
rt_rq's current task. This means a rt_rq can have a migratible current, N
non-migratible queued tasks, and be flagged as overloaded / have its CPU
set in the rto_mask, despite having an empty pushable_tasks list.

Make an rt_rq's overload logic be driven by {enqueue,dequeue}_pushable_task().
Since rt_rq->{rt_nr_migratory,rt_nr_total} become unused, remove them.

Note that the case where the current task is pushed away to make way for a
migration-disabled task remains unchanged: the migration-disabled task has
to be in the pushable_tasks list in the first place, which means it has
nr_cpus_allowed > 1.

Link: http://lore.kernel.org/r/20230801152648._y603AS_@linutronix.de
Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
This is lightly tested, this looks to be working OK but I don't have nor am
I aware of a test case for RT balancing, I suppose we want something that
asserts we always run the N highest prio tasks for N CPUs, with a small
margin for migrations?
---
 kernel/sched/debug.c |  3 --
 kernel/sched/rt.c    | 70 +++++++-------------------------------------
 kernel/sched/sched.h |  2 --
 3 files changed, 10 insertions(+), 65 deletions(-)
  

On 2023-09-11 12:54:50 [+0200], Valentin Schneider wrote:
> Ok, back to this :)
> 
> On 15/08/23 16:21, Sebastian Andrzej Siewior wrote:
> > What I still observe is:
> > - CPU0 is idle. CPU0 gets a task assigned from CPU1. That task receives
> >   a wakeup. CPU0 returns from idle and schedules the task.
> >   pull_rt_task() on CPU1 and sometimes on other CPU observe this, too.
> >   CPU1 sends irq_work to CPU0 while at the time rto_next_cpu() sees that
> >   has_pushable_tasks() return 0. That bit was cleared earlier (as per
> >   tracing).
> >
> > - CPU0 is idle. CPU0 gets a task assigned from CPU1. The task on CPU0 is
> >   woken up without an IPI (yay). But then pull_rt_task() decides that
> >   send irq_work and has_pushable_tasks() said that is has tasks left
> >   so….
> >   Now: rto_push_irq_work_func() run once once on CPU0, does nothing,
> >   rto_next_cpu() return CPU0 again and enqueues itself again on CPU0.
> >   Usually after the second or third round the scheduler on CPU0 makes
> >   enough progress to remove the task/ clear the CPU from mask.
> >
> 
> If CPU0 is selected for the push IPI, then we should have
> 
>   rd->rto_cpu == CPU0
> 
> So per the
> 
>   cpumask_next(rd->rto_cpu, rd->rto_mask);
> 
> in rto_next_cpu(), it shouldn't be able to re-select itself.
> 
> Do you have a simple enough reproducer I could use to poke at this?

Not really a reproducer. What I had earlier was a high priority RT task
(ntpsec at prio 99) and cyclictest below it (prio 90). And PREEMPT_RT
which adds a few tasks (due to threaded interrupts). 
Then I added trace-printks to observe. Initially I had latency spikes
due to ntpsec but also a bunch IRQ-work-IPIs which I decided to look at.

> > I understand that there is a race and the CPU is cleared from rto_mask
> > shortly after checking. Therefore I would suggest to look at
> > has_pushable_tasks() before returning a CPU in rto_next_cpu() as I did
> > just to avoid the interruption which does nothing.
> >
> > For the second case the irq_work seems to make no progress. I don't see
> > any trace_events in hardirq, the mask is cleared outside hardirq (idle
> > code). The NEED_RESCHED bit is set for current therefore it doesn't make
> > sense to send irq_work to reschedule if the current already has this on
> > its agenda.
> >
> > So what about something like:
> >
> > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > index 00e0e50741153..d963408855e25 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -2247,8 +2247,23 @@ static int rto_next_cpu(struct root_domain *rd)
> >
> >               rd->rto_cpu = cpu;
> >
> > -		if (cpu < nr_cpu_ids)
> > +		if (cpu < nr_cpu_ids) {
> > +			struct task_struct *t;
> > +
> > +			if (!has_pushable_tasks(cpu_rq(cpu)))
> > +				continue;
> > +
> 
> IIUC that's just to plug the race between the CPU emptying its
> pushable_tasks list and it removing itself from the rto_mask - that looks
> fine to me.
> 
> > +			rcu_read_lock();
> > +			t = rcu_dereference(rq->curr);
> > +			/* if (test_preempt_need_resched_cpu(cpu_rq(cpu))) */
> > +			if (test_tsk_need_resched(t)) {
> 
> We need to make sure this doesn't cause us to loose IPIs we actually need.
> 
> We do have a call to put_prev_task_balance() through entering __schedule()
> if the previous task is RT/DL, and balance_rt() can issue a push
> IPI, but AFAICT only if the previous task was the last DL task. So I don't
> think we can do this.

I observed that the CPU/ task on that CPU already had the need-resched
bit set so a task-switch is in progress. Therefore it looks like any
further IPIs are needless because the IRQ-work IPI just "leave early"
via resched_curr() and don't do anything useful. So they don't
contribute anything but stall the CPU from making progress and
performing the actual context switch.

> > +				rcu_read_unlock();
> > +				continue;
> > +			}
> > +			rcu_read_unlock();
> > +
> >                       return cpu;
> > +		}
> >
> >               rd->rto_cpu = -1;

Sebastian
  

* Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> On 2023-08-11 12:20:44 [+0100], Valentin Schneider wrote:
> > Sebastian noted that the rto_push_work IRQ work can be queued for a CPU
> > that has an empty pushable_tasks list, which means nothing useful will be
> > done in the IPI other than queue the work for the next CPU on the rto_mask.
> > 
> > rto_push_irq_work_func() only operates on tasks in the pushable_tasks list,
> > but the conditions for that irq_work to be queued (and for a CPU to be
> > added to the rto_mask) rely on rq_rt->nr_migratory instead.
> > 
> > nr_migratory is increased whenever an RT task entity is enqueued and it has
> > nr_cpus_allowed > 1. Unlike the pushable_tasks list, nr_migratory includes a
> > rt_rq's current task. This means a rt_rq can have a migratible current, N
> > non-migratible queued tasks, and be flagged as overloaded / have its CPU
> > set in the rto_mask, despite having an empty pushable_tasks list.
> > 
> > Make an rt_rq's overload logic be driven by {enqueue,dequeue}_pushable_task().
> > Since rt_rq->{rt_nr_migratory,rt_nr_total} become unused, remove them.
> > 
> > Note that the case where the current task is pushed away to make way for a
> > migration-disabled task remains unchanged: the migration-disabled task has
> > to be in the pushable_tasks list in the first place, which means it has
> > nr_cpus_allowed > 1.
> > 
> > Link: http://lore.kernel.org/r/20230801152648._y603AS_@linutronix.de
> > Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> > ---
> > This is lightly tested, this looks to be working OK but I don't have nor am
> > I aware of a test case for RT balancing, I suppose we want something that
> > asserts we always run the N highest prio tasks for N CPUs, with a small
> > margin for migrations?
> 
> I don't see the storm of IPIs I saw before. So as far that goes:
>    Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

I've applied Valentin's initial fix to tip:sched/core, for an eventual
v6.7 merge, as it addresses the IPI storm bug. Let me know if merging
this is not desirable for some reason.

> What I still observe is:
> - CPU0 is idle. CPU0 gets a task assigned from CPU1. That task receives
>   a wakeup. CPU0 returns from idle and schedules the task.
>   pull_rt_task() on CPU1 and sometimes on other CPU observe this, too.
>   CPU1 sends irq_work to CPU0 while at the time rto_next_cpu() sees that
>   has_pushable_tasks() return 0. That bit was cleared earlier (as per
>   tracing).
> 
> - CPU0 is idle. CPU0 gets a task assigned from CPU1. The task on CPU0 is
>   woken up without an IPI (yay). But then pull_rt_task() decides that
>   send irq_work and has_pushable_tasks() said that is has tasks left
>   so….
>   Now: rto_push_irq_work_func() run once once on CPU0, does nothing,
>   rto_next_cpu() return CPU0 again and enqueues itself again on CPU0.
>   Usually after the second or third round the scheduler on CPU0 makes
>   enough progress to remove the task/ clear the CPU from mask.

Just curious, any progress on solving this?

Thanks,

	Ingo
  

On Mon, Sep 25, 2023 at 08:55:10AM -0000, tip-bot2 for Valentin Schneider wrote:
> The following commit has been merged into the sched/core branch of tip:
> 
> Commit-ID:     612f769edd06a6e42f7cd72425488e68ddaeef0a
> Gitweb:        https://git.kernel.org/tip/612f769edd06a6e42f7cd72425488e68ddaeef0a
> Author:        Valentin Schneider <vschneid@redhat.com>
> AuthorDate:    Fri, 11 Aug 2023 12:20:44 +01:00
> Committer:     Ingo Molnar <mingo@kernel.org>
> CommitterDate: Mon, 25 Sep 2023 10:25:29 +02:00
> 
> sched/rt: Make rt_rq->pushable_tasks updates drive rto_mask
> 
> Sebastian noted that the rto_push_work IRQ work can be queued for a CPU
> that has an empty pushable_tasks list, which means nothing useful will be
> done in the IPI other than queue the work for the next CPU on the rto_mask.
> 
> rto_push_irq_work_func() only operates on tasks in the pushable_tasks list,
> but the conditions for that irq_work to be queued (and for a CPU to be
> added to the rto_mask) rely on rq_rt->nr_migratory instead.
> 
> nr_migratory is increased whenever an RT task entity is enqueued and it has
> nr_cpus_allowed > 1. Unlike the pushable_tasks list, nr_migratory includes a
> rt_rq's current task. This means a rt_rq can have a migratible current, N
> non-migratible queued tasks, and be flagged as overloaded / have its CPU
> set in the rto_mask, despite having an empty pushable_tasks list.
> 
> Make an rt_rq's overload logic be driven by {enqueue,dequeue}_pushable_task().
> Since rt_rq->{rt_nr_migratory,rt_nr_total} become unused, remove them.
> 
> Note that the case where the current task is pushed away to make way for a
> migration-disabled task remains unchanged: the migration-disabled task has
> to be in the pushable_tasks list in the first place, which means it has
> nr_cpus_allowed > 1.
> 
> Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Link: https://lore.kernel.org/r/20230811112044.3302588-1-vschneid@redhat.com
> ---
>  kernel/sched/debug.c |  3 +--
>  kernel/sched/rt.c    | 70 ++++++-------------------------------------
>  kernel/sched/sched.h |  2 +-
>  3 files changed, 10 insertions(+), 65 deletions(-)
> 

> @@ -358,53 +357,6 @@ static inline void rt_clear_overload(struct rq *rq)
>  	cpumask_clear_cpu(rq->cpu, rq->rd->rto_mask);
>  }
>  
> -static void update_rt_migration(struct rt_rq *rt_rq)
> -{
> -	if (rt_rq->rt_nr_migratory && rt_rq->rt_nr_total > 1) {
> -		if (!rt_rq->overloaded) {
> -			rt_set_overload(rq_of_rt_rq(rt_rq));
> -			rt_rq->overloaded = 1;
> -		}
> -	} else if (rt_rq->overloaded) {
> -		rt_clear_overload(rq_of_rt_rq(rt_rq));
> -		rt_rq->overloaded = 0;
> -	}
> -}
> -
> -static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> -{
> -	struct task_struct *p;
> -
> -	if (!rt_entity_is_task(rt_se))
> -		return;
> -
> -	p = rt_task_of(rt_se);
> -	rt_rq = &rq_of_rt_rq(rt_rq)->rt;
> -
> -	rt_rq->rt_nr_total++;
> -	if (p->nr_cpus_allowed > 1)
> -		rt_rq->rt_nr_migratory++;
> -
> -	update_rt_migration(rt_rq);
> -}
> -
> -static void dec_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> -{
> -	struct task_struct *p;
> -
> -	if (!rt_entity_is_task(rt_se))
> -		return;
> -
> -	p = rt_task_of(rt_se);
> -	rt_rq = &rq_of_rt_rq(rt_rq)->rt;
> -
> -	rt_rq->rt_nr_total--;
> -	if (p->nr_cpus_allowed > 1)
> -		rt_rq->rt_nr_migratory--;
> -
> -	update_rt_migration(rt_rq);
> -}

sched/deadline.c has something very similar, does that need updating
too?
  

On 25/09/23 10:27, Ingo Molnar wrote:
> * Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
>
>> On 2023-08-11 12:20:44 [+0100], Valentin Schneider wrote:
>> > Sebastian noted that the rto_push_work IRQ work can be queued for a CPU
>> > that has an empty pushable_tasks list, which means nothing useful will be
>> > done in the IPI other than queue the work for the next CPU on the rto_mask.
>> > 
>> > rto_push_irq_work_func() only operates on tasks in the pushable_tasks list,
>> > but the conditions for that irq_work to be queued (and for a CPU to be
>> > added to the rto_mask) rely on rq_rt->nr_migratory instead.
>> > 
>> > nr_migratory is increased whenever an RT task entity is enqueued and it has
>> > nr_cpus_allowed > 1. Unlike the pushable_tasks list, nr_migratory includes a
>> > rt_rq's current task. This means a rt_rq can have a migratible current, N
>> > non-migratible queued tasks, and be flagged as overloaded / have its CPU
>> > set in the rto_mask, despite having an empty pushable_tasks list.
>> > 
>> > Make an rt_rq's overload logic be driven by {enqueue,dequeue}_pushable_task().
>> > Since rt_rq->{rt_nr_migratory,rt_nr_total} become unused, remove them.
>> > 
>> > Note that the case where the current task is pushed away to make way for a
>> > migration-disabled task remains unchanged: the migration-disabled task has
>> > to be in the pushable_tasks list in the first place, which means it has
>> > nr_cpus_allowed > 1.
>> > 
>> > Link: http://lore.kernel.org/r/20230801152648._y603AS_@linutronix.de
>> > Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>> > Signed-off-by: Valentin Schneider <vschneid@redhat.com>
>> > ---
>> > This is lightly tested, this looks to be working OK but I don't have nor am
>> > I aware of a test case for RT balancing, I suppose we want something that
>> > asserts we always run the N highest prio tasks for N CPUs, with a small
>> > margin for migrations?
>> 
>> I don't see the storm of IPIs I saw before. So as far that goes:
>>    Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>
> I've applied Valentin's initial fix to tip:sched/core, for an eventual
> v6.7 merge, as it addresses the IPI storm bug. Let me know if merging
> this is not desirable for some reason.
>
>> What I still observe is:
>> - CPU0 is idle. CPU0 gets a task assigned from CPU1. That task receives
>>   a wakeup. CPU0 returns from idle and schedules the task.
>>   pull_rt_task() on CPU1 and sometimes on other CPU observe this, too.
>>   CPU1 sends irq_work to CPU0 while at the time rto_next_cpu() sees that
>>   has_pushable_tasks() return 0. That bit was cleared earlier (as per
>>   tracing).
>> 
>> - CPU0 is idle. CPU0 gets a task assigned from CPU1. The task on CPU0 is
>>   woken up without an IPI (yay). But then pull_rt_task() decides that
>>   send irq_work and has_pushable_tasks() said that is has tasks left
>>   so….
>>   Now: rto_push_irq_work_func() run once once on CPU0, does nothing,
>>   rto_next_cpu() return CPU0 again and enqueues itself again on CPU0.
>>   Usually after the second or third round the scheduler on CPU0 makes
>>   enough progress to remove the task/ clear the CPU from mask.
>
> Just curious, any progress on solving this?
>

On my side not really, I need to stop getting distracted and probably get
this to reproduce on a system so I can understand-by-tracing

> Thanks,
>
> 	Ingo
  

On 25/09/23 12:11, Peter Zijlstra wrote:
> On Mon, Sep 25, 2023 at 08:55:10AM -0000, tip-bot2 for Valentin Schneider wrote:
>> The following commit has been merged into the sched/core branch of tip:
>> 
>> Commit-ID:     612f769edd06a6e42f7cd72425488e68ddaeef0a
>> Gitweb:        https://git.kernel.org/tip/612f769edd06a6e42f7cd72425488e68ddaeef0a
>> Author:        Valentin Schneider <vschneid@redhat.com>
>> AuthorDate:    Fri, 11 Aug 2023 12:20:44 +01:00
>> Committer:     Ingo Molnar <mingo@kernel.org>
>> CommitterDate: Mon, 25 Sep 2023 10:25:29 +02:00
>> 
>> sched/rt: Make rt_rq->pushable_tasks updates drive rto_mask
>> 
>> Sebastian noted that the rto_push_work IRQ work can be queued for a CPU
>> that has an empty pushable_tasks list, which means nothing useful will be
>> done in the IPI other than queue the work for the next CPU on the rto_mask.
>> 
>> rto_push_irq_work_func() only operates on tasks in the pushable_tasks list,
>> but the conditions for that irq_work to be queued (and for a CPU to be
>> added to the rto_mask) rely on rq_rt->nr_migratory instead.
>> 
>> nr_migratory is increased whenever an RT task entity is enqueued and it has
>> nr_cpus_allowed > 1. Unlike the pushable_tasks list, nr_migratory includes a
>> rt_rq's current task. This means a rt_rq can have a migratible current, N
>> non-migratible queued tasks, and be flagged as overloaded / have its CPU
>> set in the rto_mask, despite having an empty pushable_tasks list.
>> 
>> Make an rt_rq's overload logic be driven by {enqueue,dequeue}_pushable_task().
>> Since rt_rq->{rt_nr_migratory,rt_nr_total} become unused, remove them.
>> 
>> Note that the case where the current task is pushed away to make way for a
>> migration-disabled task remains unchanged: the migration-disabled task has
>> to be in the pushable_tasks list in the first place, which means it has
>> nr_cpus_allowed > 1.
>> 
>> Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
>> Signed-off-by: Ingo Molnar <mingo@kernel.org>
>> Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>> Link: https://lore.kernel.org/r/20230811112044.3302588-1-vschneid@redhat.com
>> ---
>>  kernel/sched/debug.c |  3 +--
>>  kernel/sched/rt.c    | 70 ++++++-------------------------------------
>>  kernel/sched/sched.h |  2 +-
>>  3 files changed, 10 insertions(+), 65 deletions(-)
>> 
>
>> @@ -358,53 +357,6 @@ static inline void rt_clear_overload(struct rq *rq)
>>  	cpumask_clear_cpu(rq->cpu, rq->rd->rto_mask);
>>  }
>>  
>> -static void update_rt_migration(struct rt_rq *rt_rq)
>> -{
>> -	if (rt_rq->rt_nr_migratory && rt_rq->rt_nr_total > 1) {
>> -		if (!rt_rq->overloaded) {
>> -			rt_set_overload(rq_of_rt_rq(rt_rq));
>> -			rt_rq->overloaded = 1;
>> -		}
>> -	} else if (rt_rq->overloaded) {
>> -		rt_clear_overload(rq_of_rt_rq(rt_rq));
>> -		rt_rq->overloaded = 0;
>> -	}
>> -}
>> -
>> -static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
>> -{
>> -	struct task_struct *p;
>> -
>> -	if (!rt_entity_is_task(rt_se))
>> -		return;
>> -
>> -	p = rt_task_of(rt_se);
>> -	rt_rq = &rq_of_rt_rq(rt_rq)->rt;
>> -
>> -	rt_rq->rt_nr_total++;
>> -	if (p->nr_cpus_allowed > 1)
>> -		rt_rq->rt_nr_migratory++;
>> -
>> -	update_rt_migration(rt_rq);
>> -}
>> -
>> -static void dec_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
>> -{
>> -	struct task_struct *p;
>> -
>> -	if (!rt_entity_is_task(rt_se))
>> -		return;
>> -
>> -	p = rt_task_of(rt_se);
>> -	rt_rq = &rq_of_rt_rq(rt_rq)->rt;
>> -
>> -	rt_rq->rt_nr_total--;
>> -	if (p->nr_cpus_allowed > 1)
>> -		rt_rq->rt_nr_migratory--;
>> -
>> -	update_rt_migration(rt_rq);
>> -}
>
> sched/deadline.c has something very similar, does that need updating
> too?

Hm I think so yes:
- push_dl_task() is an obvious noop if the pushable tree is empty
- pull_dl_task() can be kicked if !rq->dl.overloaded, which similarly to rt
  is driven by nr_migratory but could be boiled down to having pushable
  tasks (due to the nr_cpus_allowed > 1 constraint).

Lemme poke at it.
  

* Valentin Schneider <vschneid@redhat.com> wrote:

> > sched/deadline.c has something very similar, does that need updating 
> > too?
> 
> Hm I think so yes:
> - push_dl_task() is an obvious noop if the pushable tree is empty
> - pull_dl_task() can be kicked if !rq->dl.overloaded, which similarly to rt
>   is driven by nr_migratory but could be boiled down to having pushable
>   tasks (due to the nr_cpus_allowed > 1 constraint).
> 
> Lemme poke at it.

For reference, the matching DL change now lives in tip:sched/core as:

   177f608a4c82 ("sched/deadline: Make dl_rq->pushable_dl_tasks update drive dl_rq->overloaded")

Thanks,

	Ingo