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    SubjectRe: Why can't I set the priority of softirq-hrt? (Re: 2.6.17-rt1)

    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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