Re: [patch] SCHED_SOFTRR starve-free linux scheduling policy ...

[Daniel Phillips <phillips@arcor.de>]

On Saturday 09 August 2003 18:47, Mike Galbraith wrote:
> > But the patch has a much bigger problem: there is no way a SOFTRR task can
> > be realtime as long as higher priority non-realtime tasks can preempt it.
> > The new dynamic priority adjustment makes it certain that we will
> > regularly see normal tasks with priority elevated above so-called
> > realtime tasks.  Even without dynamic priority adjustment, any higher
> > priority system task can unwttingly make a mockery of realtime schedules.
>
> Not so.

Yes so.  A SCHED_NORMAL task with priority n can execute even when a 
SCHED_FIFO/RR/SOFTRR task of priority n-1 is ready.  In the case of FIFO and 
RR we don't care because they're already unusable by normal users but in the 
case of SOFTRR it defeats the intended realtime gaurantee.

> Dynamic priority adjustment will not put a SCHED_OTHER task above
> SCHED_RR, SCHED_FIFO or SCHED_SOFTRR, so they won't preempt.

Are you sure?  I suppose that depends on the particular flavor of dynamic 
priority adjustment.  The last I saw, dynamic priority can adjust the task 
priority by 5 up or down.  If I'm wrong, please show me why and hopefully 
point at specific code.

On the other hand, this doesn't really matter, because the first problem above 
still exists, and causes the same result: attempted realtime scheduling that 
isn't.

Regards,

Daniel