Messages in this thread | | | Subject | Re: RT task scheduling | From | Steven Rostedt <> | Date | Sat, 08 Apr 2006 00:45:49 -0400 |
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Hi Vernon,
On Fri, 2006-04-07 at 21:28 -0700, Vernon Mauery wrote:
> 1) Deterministic scheduling algorithms (SWSRPS). Basically, with uniprocessor > systems (or smp with a global run queue), it was really easy to say, run the > highest priority task in the queue. But when there are several queues that > are independent of each other, it is difficult. According to SWSRPS, nr_cpus > highest priority runnable tasks should _always_ be running (regardless of > which queue they are on). This might mean that there are longer latencies a) > to determine the nr_cpus highest priority tasks and b) because of cache > issues.
Yep, and task cpu dancing. Everytime a High prio task preempts a lower prio RT task, that RT task might be pushed to another CPU.
> > 2) Maximum deterministic latency. A task should be able to say that if it > relinquishes the processor for now, MAX_LATENCY nanoseconds (or ticks or > whatever you want to measure time in) later, it will be back in time to meet > a deadline.
Yep, but the more important thing than latency, is to make your deadline. Sometimes people forget that and just concentrate on latency. But that's another story.
> > As I understand it, real time is all about determinism. But there are several > places where we have to focus on determinism to make it all behave as it > should. > > Priority A > B > C > If a lower priority task C gets run just because it is the highest in that > CPU's run queue while there is a higher priority task B is sleeping while A > runs (on a 2 proc system), this is WRONG.
Argh, terminology is killing us all. For this to be wrong, B isn't "sleeping" it's "waiting" while in the run state. "Sleeping" means that it's not on the run queue and is just waiting for some event. Which would be OK for C to run then. But if B is on the run queue and in the the TASK_RUNNING state, it would be wrong for C to be running somewhere where B could be running.
> But then again, we need to make > sure that we can determine the maximum latency to preempt C to run B and try > to minimize that.
And here I don't know of another way besides an IPI to preempt C. If C is in userspace, how would you preempt C right a way if B suddenly wakes up on the runqueue of A?
> > Poof! More smoke in the air. I hope that clears it up.
It's as clear as my face was in High School ;)
-- Steve
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