Messages in this thread | | | Date | Tue, 20 Jun 2006 12:39:38 -0400 (EDT) | From | Steven Rostedt <> | Subject | Re: Why can't I set the priority of softirq-hrt? (Re: 2.6.17-rt1) |
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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?
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.
> > 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.
> > 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.
-- Steve
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