[PATCH] sched_rt_rq_enqueue() resched idle

[PATCH] sched_rt_rq_enqueue() resched idle

[John Blackwood]

Hi Peter,

When sysctl_sched_rt_runtime is set to something other than -1 and the
CONFIG_RT_GROUP_SCHED kernel parameter is NOT enabled, we get into a state
where we see one or more CPUs idling forvever even though there are
real-time
tasks in their rt runqueue that are able to run (no longer throttled).

The sequence is:

- A real-time task is running when the timer sets the rt runqueue
    to throttled, and the rt task is resched_task()ed and switched
    out, and idle is switched in since there are no non-rt tasks to
    run on that cpu.

- Eventually the do_sched_rt_period_timer() runs and un-throttles
    the rt runqueue, but we just exit the timer interrupt and go back
    to executing the idle task in the idle loop forever.

If we change the sched_rt_rq_enqueue() routine to use some of the code
from the CONFIG_RT_GROUP_SCHED enabled version of this same routine and
resched_task() the currently executing task (idle in our case) if it is
a lower priority task than the higher rt task in the now un-throttled
runqueue, the problem is no longer observed.

Thank you for your time and consideration.

Signed-off-by: John Blackwood <john.blackwood@ccur.com>

Index: a/kernel/sched_rt.c
===================================================================
--- a.orig/kernel/sched_rt.c    2008-08-22 15:11:31.000000000 -0400
+++ a/kernel/sched_rt.c 2008-08-22 15:12:36.000000000 -0400
@@ -193,6 +193,12 @@

   static inline void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
   {
+       if (rt_rq->rt_nr_running) {
+               struct task_struct *curr = rq_of_rt_rq(rt_rq)->curr;
+
+               if (rt_rq->highest_prio < curr->prio)
+                       resched_task(curr);
+       }
   }

   static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq)

Re: [PATCH] sched_rt_rq_enqueue() resched idle

[Peter Zijlstra]

On Tue, 2008-08-26 at 15:09 -0400, John Blackwood wrote:
> Hi Peter,
> 
> When sysctl_sched_rt_runtime is set to something other than -1 and the
> CONFIG_RT_GROUP_SCHED kernel parameter is NOT enabled, we get into a state
> where we see one or more CPUs idling forvever even though there are
> real-time
> tasks in their rt runqueue that are able to run (no longer throttled).
> 
> The sequence is:
> 
> - A real-time task is running when the timer sets the rt runqueue
>     to throttled, and the rt task is resched_task()ed and switched
>     out, and idle is switched in since there are no non-rt tasks to
>     run on that cpu.
> 
> - Eventually the do_sched_rt_period_timer() runs and un-throttles
>     the rt runqueue, but we just exit the timer interrupt and go back
>     to executing the idle task in the idle loop forever.
> 
> If we change the sched_rt_rq_enqueue() routine to use some of the code
> from the CONFIG_RT_GROUP_SCHED enabled version of this same routine and
> resched_task() the currently executing task (idle in our case) if it is
> a lower priority task than the higher rt task in the now un-throttled
> runqueue, the problem is no longer observed.

Very good spotting, Thanks!

However I think the patch isn't quite good, as highest_prio is only
available on SMP || RT_GROUP_SCHED.

Furthermore, on !RT_GROUP_SCHED any RT task will be higher than current,
so we can do the below, do you agree?

---
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 94daace..f672aee 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -199,6 +199,8 @@ static inline struct rt_rq *group_rt_rq(struct sched_rt_entity *rt_se)
 
 static inline void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
 {
+	if (rt_rq->rt_nr_running)
+		resched_task(rq_of_rt_rq(rt_rq)->curr);
 }
 
 static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq)

Re: [PATCH] sched_rt_rq_enqueue() resched idle

[John Blackwood]

 > On Tue, 2008-08-26 at 15:09 -0400, John Blackwood wrote:
 > > > Hi Peter,
 > > >
 > > > When sysctl_sched_rt_runtime is set to something other than -1 
and the
 > > > CONFIG_RT_GROUP_SCHED kernel parameter is NOT enabled, we get 
into a state
 > > > where we see one or more CPUs idling forvever even though there are
 > > > real-time
 > > > tasks in their rt runqueue that are able to run (no longer 
throttled).
 > > >
 > > > The sequence is:
 > > >
 > > > - A real-time task is running when the timer sets the rt runqueue
 > > >     to throttled, and the rt task is resched_task()ed and switched
 > > >     out, and idle is switched in since there are no non-rt tasks to
 > > >     run on that cpu.
 > > >
 > > > - Eventually the do_sched_rt_period_timer() runs and un-throttles
 > > >     the rt runqueue, but we just exit the timer interrupt and go back
 > > >     to executing the idle task in the idle loop forever.
 > > >
 > > > If we change the sched_rt_rq_enqueue() routine to use some of the 
code
 > > > from the CONFIG_RT_GROUP_SCHED enabled version of this same 
routine and
 > > > resched_task() the currently executing task (idle in our case) if 
it is
 > > > a lower priority task than the higher rt task in the now un-throttled
 > > > runqueue, the problem is no longer observed.
 >
 > Very good spotting, Thanks!

You're welcome.

 > However I think the patch isn't quite good, as highest_prio is only
 > available on SMP || RT_GROUP_SCHED.
 >
 > Furthermore, on !RT_GROUP_SCHED any RT task will be higher than current,
 > so we can do the below, do you agree?

Yes, I see what you are saying.
The patch version below looks good.
I re-tested with it and it works fine.

Thanks!

 > diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
 > index 94daace..f672aee 100644
 > --- a/kernel/sched_rt.c
 > +++ b/kernel/sched_rt.c
 > @@ -199,6 +199,8 @@ static inline struct rt_rq *group_rt_rq(struct 
sched_rt_entity *rt_se)
 >
 >  static inline void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
 >  {
 > +       if (rt_rq->rt_nr_running)
 > +               resched_task(rq_of_rt_rq(rt_rq)->curr);
 >  }
 >
 >  static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq)
 >
 >

Re: [PATCH] sched_rt_rq_enqueue() resched idle

[Peter Zijlstra]

Ingo, please apply.


---
Subject: sched: sched_rt_rq_enqueue() resched idle
From: John Blackwood <john.blackwood@ccur.com>
Date: Tue, 26 Aug 2008 15:09:43 -0400

When sysctl_sched_rt_runtime is set to something other than -1 and the
CONFIG_RT_GROUP_SCHED kernel parameter is NOT enabled, we get into a state
where we see one or more CPUs idling forvever even though there are
real-time
tasks in their rt runqueue that are able to run (no longer throttled).

The sequence is:

- A real-time task is running when the timer sets the rt runqueue
    to throttled, and the rt task is resched_task()ed and switched
    out, and idle is switched in since there are no non-rt tasks to
    run on that cpu.

- Eventually the do_sched_rt_period_timer() runs and un-throttles
    the rt runqueue, but we just exit the timer interrupt and go back
    to executing the idle task in the idle loop forever.

If we change the sched_rt_rq_enqueue() routine to use some of the code
from the CONFIG_RT_GROUP_SCHED enabled version of this same routine and
resched_task() the currently executing task (idle in our case) if it is
a lower priority task than the higher rt task in the now un-throttled
runqueue, the problem is no longer observed.

Signed-off-by: John Blackwood <john.blackwood@ccur.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 kernel/sched_rt.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-2.6/kernel/sched_rt.c
===================================================================
--- linux-2.6.orig/kernel/sched_rt.c
+++ linux-2.6/kernel/sched_rt.c
@@ -199,6 +199,8 @@ static inline struct rt_rq *group_rt_rq(
 
 static inline void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
 {
+	if (rt_rq->rt_nr_running)
+		resched_task(rq_of_rt_rq(rt_rq)->curr);
 }
 
 static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq)