Scheduler RR, first time slice wrong?

Scheduler RR, first time slice wrong?

[Andreas Huber]

Hi,

when starting a real-time process with the round-robin scheduler, the
first time slice is set to HZ (1 second).
When starting two such processes at the same time of the same static
priority, the second one started is first executed after 1 second.
(supposing the first process is heavy on cpu load).
After both have exhausted this first time slice, it is set to
DEF_TIMESLICE (which is 100ms).

Is this behavior as it is supposed to be?

Please add me in CC.

Regards,
Andreas


Relevant parts in the code:
include/linux/init_task.h
#define INIT_TASK(tsk) \
	.rt	= { \
		.time_slice = HZ,

kernel/sched.c
#define DEF_TIMESLICE   (100 * HZ / 1000)

kernel/sched_rt.c
static void task_tick_rt(struct rq *rq, struct task_struct *p, int
queued)
  if (--p->rt.time_slice)
    return;

  p->rt.time_slice = DEF_TIMESLICE;

Re: Scheduler RR, first time slice wrong?

[Ingo Molnar]

(added Cc:s)

* Andreas Huber <andreas.huber@keymile.com> wrote:

> Hi,
> 
> when starting a real-time process with the round-robin scheduler, the
> first time slice is set to HZ (1 second).
> When starting two such processes at the same time of the same static
> priority, the second one started is first executed after 1 second.
> (supposing the first process is heavy on cpu load).
> After both have exhausted this first time slice, it is set to
> DEF_TIMESLICE (which is 100ms).
> 
> Is this behavior as it is supposed to be?
> 
> Please add me in CC.
> 
> Regards,
> Andreas
> 
> 
> Relevant parts in the code:
> include/linux/init_task.h
> #define INIT_TASK(tsk) \
> 	.rt	= { \
> 		.time_slice = HZ,
> 
> kernel/sched.c
> #define DEF_TIMESLICE   (100 * HZ / 1000)
> 
> kernel/sched_rt.c
> static void task_tick_rt(struct rq *rq, struct task_struct *p, int
> queued)
>   if (--p->rt.time_slice)
>     return;
> 
>   p->rt.time_slice = DEF_TIMESLICE;
> 
> --
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Re: Scheduler RR, first time slice wrong?

[Peter Zijlstra]

On Thu, 2008-11-06 at 08:44 +0100, Ingo Molnar wrote:
> (added Cc:s)
> 
> * Andreas Huber <andreas.huber@keymile.com> wrote:
> 
> > Hi,
> > 
> > when starting a real-time process with the round-robin scheduler, the
> > first time slice is set to HZ (1 second).
> > When starting two such processes at the same time of the same static
> > priority, the second one started is first executed after 1 second.
> > (supposing the first process is heavy on cpu load).
> > After both have exhausted this first time slice, it is set to
> > DEF_TIMESLICE (which is 100ms).
> > 
> > Is this behavior as it is supposed to be?
> > 
> > Please add me in CC.
> > 
> > Regards,
> > Andreas
> > 
> > 
> > Relevant parts in the code:
> > include/linux/init_task.h
> > #define INIT_TASK(tsk) \
> > 	.rt	= { \
> > 		.time_slice = HZ,
> > 
> > kernel/sched.c
> > #define DEF_TIMESLICE   (100 * HZ / 1000)
> > 
> > kernel/sched_rt.c
> > static void task_tick_rt(struct rq *rq, struct task_struct *p, int
> > queued)
> >   if (--p->rt.time_slice)
> >     return;
> > 
> >   p->rt.time_slice = DEF_TIMESLICE;
> > 

How does this work?


---
Subject: sched: reset RR timeslice on fork

Currently we inherit the RR time-slice from our parent. In case that is
init we inherit the 1s value set by default, in case its an already
running RR task, we inherit whatever is left.

Fix both these by resetting the time-slice on fork.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
diff --git a/kernel/sched.c b/kernel/sched.c
index 241fd85..e7d69a0 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2337,6 +2337,7 @@ static void __sched_fork(struct task_struct *p)
 #endif
 
 	INIT_LIST_HEAD(&p->rt.run_list);
+	p->rt.time_slice = DEF_TIMESLICE;
 	p->se.on_rq = 0;
 	INIT_LIST_HEAD(&p->se.group_node);
 

Re: Scheduler RR, first time slice wrong?

[Ingo Oeser]

Hi Peter,

On Thursday 06 November 2008, Peter Zijlstra wrote:
> Currently we inherit the RR time-slice from our parent. In case that is
> init we inherit the 1s value set by default, in case its an already
> running RR task, we inherit whatever is left.
> 
> Fix both these by resetting the time-slice on fork.

I remember that this MIGHT be on purpose to mitigate fork bombs.

Behaviour was, that you could starve others, if you fork, 
do little or no work and fork again before your timeslice is over.

But I'm not sure this applies here...


Best Regards

Ingo Oeser

Re: Scheduler RR, first time slice wrong?

[Peter Zijlstra]

On Fri, 2008-11-07 at 10:19 +0100, Ingo Oeser wrote:
> Hi Peter,
> 
> On Thursday 06 November 2008, Peter Zijlstra wrote:
> > Currently we inherit the RR time-slice from our parent. In case that is
> > init we inherit the 1s value set by default, in case its an already
> > running RR task, we inherit whatever is left.
> > 
> > Fix both these by resetting the time-slice on fork.
> 
> I remember that this MIGHT be on purpose to mitigate fork bombs.
> 
> Behaviour was, that you could starve others, if you fork, 
> do little or no work and fork again before your timeslice is over.
> 
> But I'm not sure this applies here...

Right, RT tasks have it easy starving stuff ;-)