[LITMUS^RT] Schedulability checking

[Björn Brandenburg]

On Feb 17, 2012, at 6:03 PM, Felipe Cerqueira wrote:
> 
> Litmus-RT papers usually include schedulability comparisons that use schedulability tests and tasks inflated with overhead costs.
> Is it possible to check schedulability by actually running the tasks?
> 
> I've been trying to modify rtspin to do that, but I'm facing some problems.
> 
> […]
> Has anyone already done schedulability checking inside the application? I thought it would be easier to do than embedding overhead costs within the tasks.
> In case it is a bad approach, I can try the usual way.


I think your approach fundamentally doesn't work. Schedulability is an analytical property. A task set is schedulable if it can be shown that it won't miss a deadline (in the hard real-time case) for *any* legal arrival sequence. Observing whether deadlines are missed in the task can only establish that it is *not* schedulable, but not the inverse.

By the way, rtpsin spins for approximately 90% of its configured WCET because the delay loop isn't exact. It may overshoot a little; hence setting the delay to 100% leads to the problems that you've described.

- Björn