Messages in this thread | | | Date | Tue, 20 Jun 2006 19:12:51 +0100 (BST) | Subject | Re: Why can't I set the priority of softirq-hrt? (Re: 2.6.17-rt1) | From | Esben Nielsen <> |
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On Tue, 20 Jun 2006, Steven Rostedt wrote:
> > On Tue, 20 Jun 2006, Esben Nielsen wrote: > >> I am sorry. I should have read some more of the code before asking. >> >> The only question I have is why the priority of the callback is set to >> priority of the task calling hrtimer_start() (current->normal_prio). That >> seems like an odd binding to me. Shouldn't the finding of the priority be moved over to the >> posix-timer code, where it is needed, and be given as a parameter to >> hrtimer_start()? >> In rtmutex.c, where a hrtimer is used as a timeout on a mutex, wouldn't it >> make more sense to use current->prio than current->normal_prio if the task >> is boosted when it starts to wait on a mutex. > > That seems reasonable. It probably is a bug to use normal_prio, since we > really do care what prio is at that time. > >> >> >> But I am not sure I like the design at all: >> >> Let say you have a bunch of callback running at priority 1 and then the >> next hrt timer with priority 99 expires. Then the callback which >> is running will be boosted to priority 99. So the overall latency at >> priority 99 will at least the latency of the worst hrtimer callback. > > You mean for those that expire at the same time? >
No, when the priority 1 (userspace prio) expires just before the priority 99.
> I don't think this is a problem, because the run_hrtimer_hres_queue runs > the hightest priorty callback first, then it adjusts its prio to the next > priority callback. See hrtimer_adjust_softirq_prio. > >> And worse: What if the callback running is blocked on a mutex? Will the >> owner of the mutex be boosted as well? Not according to the code in >> sched.c. Therefore you get priority inversion to priority 1. That is the >> worst case hrtimer latency is that of priority 1. > > I don't see this.
Look at this situation: softirq-hrt, running some callback, is priority 1 (US prio as always) blocked for a mutex owned by some task, A. This now have priority 1 (in the worst case).The HRT interrupt comes and calls setscheduler(... prio 99). That doesn't change the priority of task A as far as I can see from the code. So in effect the priority 99 callback will wait for task A which is still priority 1. That is a priority inversion.
> >> >> Therefore, a simpler and more robust design would be to give the thread >> priority 99 as a default - just as the posix_cpu_timer thread. Then the >> system designer can move it around with chrt when needed. >> In fact you can say the current design have both the worst cases of having >> it running as priority 99 and at priority 1! > > I still don't see this happening.
The two worst cases are: 1) The system wide system 99 worst case latency is at least that of the longest callback. 2) The worst case latency of softirq-hrt is that of priority 1.
If you could set it by chrt you could at least choose which evil thing you want.
> >> >> Another complicated design would be to make a task for each priority. >> Then the interrupt wakes the highest priority one, which handles the first >> callback and awakes the next one etc. > > Don't think that is necessary.
Me neither :-) Running sofhtirq-hrt at priority 99 - or whatever is set by chrt - should be sufficient.
> > -- Steve >
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