Messages in this thread | | | Date | Sat, 9 Oct 2004 12:46:00 +0200 | From | Ingo Molnar <> | Subject | Re: voluntary-preempt-2.6.9-rc3-mm3-T3 |
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* Lee Revell <rlrevell@joe-job.com> wrote:
> > > With VP and PREEMPT in general, does the scheduler always run the > > > highest priority process, or do we only preempt if a SCHED_FIFO process > > > is runnable? > > > > Always the highest priority runnable. > > > > Hmm, interesting. Would there be any advantage to a mode where only > SCHED_FIFO tasks can preempt? This seems like a much lighter way to > solve the realtime problem.
it could be done, but i dont think we should do it. It makes RT scheduling much more of a special-case. Right now RT scheduling is 99% like normal scheduling - with the difference that RT priorities are "higher" than the normal priorities and that each RT priority level is "exclusive": the scheduler will let such tasks run until they want, without applying fairness policies.
by making RT tasks more of a special case we'd destabilize the whole thing: there would be kernel preemptability bugs that only RT tasks would hit - resulting in a steady deterioration of PREEMPT support in the kernel. (the ratio of RT tasks is perhaps 0.1% of all use, or less.) So applying _any_ RT-only technique besides the bare minimum is asking for trouble in the long run.
furthermore, we had hard-to-trigger SMP bugs that the PREEMPT kernel triggered much faster - resulting in an indirect stabilization of our SMP code. If nothing else then this alone makes PREEMPT very useful.
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