Re: [PATCH]sched: Isochronous class v2 for unprivileged softrt scheduling

[utz lehmann]

On Sat, 2005-01-22 at 10:48 +1100, Con Kolivas wrote:
> utz lehmann wrote:
> > Hi
> > 
> > I dislike the behavior of the SCHED_ISO patch that iso tasks are
> > degraded to SCHED_NORMAL if they exceed the limit.
> > IMHO it's better to throttle them at the iso_cpu limit.
> > 
> > I have modified Con's iso2 patch to do this. If iso_cpu > 50 iso tasks
> > only get stalled for 1 tick (1ms on x86).
> 
> Some tasks are so cache intensive they would make almost no forward 
> progress running for only 1ms.

Ok. The throttle duration can be exceed.
What is a good value? 5ms, 10ms?
 
> 
> > Fortunately there is a currently unused task prio (MAX_RT_PRIO-1) [1]. I
> 
> Your implementation is not correct. The "prio" field of real time tasks 
> is determined by MAX_RT_PRIO-1-rt_priority. Therefore you're limiting 
> the best real time priority, not the other way around.

Really? The task prios are (lower value is higher priority):

0
..			For SCHED_FIFO/SCHED_RR (rt_priority 99..1)
98	MAX_RT_PRIO-2

99	MAX_RT_PRIO-1 	ISO_PRIO (rt_priority 0)	

100	MAX_RT_PRIO
..			For SCHED_NORMAL
139	MAX_PRIO-1

ISO_PRIO is between the SCHED_FIFO/SCHED_RR and the SCHED_NORMAL range.

> 
> Throttling them for only 1ms will make it very easy to starve the system 
>   with 1 or more short running (<1ms) SCHED_NORMAL tasks running. Lower 
> priority tasks will never run.
> 
> Cheers,
> Con

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