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

[Mike Galbraith]

At 01:41 AM 8/10/2003 +0100, Daniel Phillips wrote:
> 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.

No, _not_ so.  How is the SCHED_NORMAL (didn't that used to be called 
SCHED_OTHER?) task ever going to receive the cpu when a realtime task is 
runnable given that 1. task selection is done via sched_find_first_bit(), 
and 2. realtime queues reside at the top of the array?

> > 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.

See the definition of rt_task() in sched.c, and the comments in sched.h 
beginning at line 266.

        -Mike 

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