Re: SCHED_RR/SCHED_FIFO and kernel threads?

[Patrik Hägglund]

Steven Rostedt wrote:

> On Fri, 2005-06-17 at 08:38 +0200, Patrik Hägglund wrote:
>  
>
> > Don't you get the problem with priority inversion? I.e., if you have two 
> > processes, P1 and P2, scheduled with SCHED_FIFO, where P1 has higer 
> > priority than P2. Now, if P1 gets blocked and needs some kernel thread 
> > to execute to get unblocked, then P2 is scheduled before the kernel 
> > thread, and can execute without any time limit.
> >    
>
> Yep, that could happen.
>
>  
>
> > That is, you should be much better off if the kernel threads has a 
> > _high_ priority. Then the execution progress can only be blocked by 
> > kernel threads, not by user space threads and processes. Or have I 
> > missed something?
> >    
>
> Still have that problem with priority inversion. Kernel threads share
> date structures with user processes (when they are in kernel mode) and
> that kernel thread that is needed may get blocked on a process that is
> lower in priority than the two mentioned above.
>
>  
>
> > (Besides that, as I see it, SCHED_RR/SCHED_FIFO are scheduling 
> > abstractions on their own, not necessarily  connected to  "low latency " 
> > or "realtime".)
> >    
>
> Only in the vanilla kernel. See Ingo's RT work. It handles priority
> inversion and SCHED_RR/SCHED_FIFO are actually connected to "low
> latency" and "realtime".
>
> http://people.redhat.com/mingo/realtime-preempt/
>
> -- Steve
>
>
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>  
Thanks for the pointer to Ingo's work. I will have a look.

Regarding my last comment: What I was trying to say was that I thought 
there are _basic_ aspects to consider (i.e. my original problem with 
kernel starvation) when implementing SCHED_RR/SCHED_FIFO _before_ you 
consider how to implement them in a "low latency" or "realtime" context.

/Patrik Hägglund
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