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SubjectRe: [RT] rt priority losing
On Mon, 24 Jul 2006, Steven Rostedt wrote:

> On Mon, 2006-07-24 at 18:00 +0100, Esben Nielsen wrote:
>> On Mon, 24 Jul 2006, Steven Rostedt wrote:
>>> Ingo or Tglx,
>>> It has come to my attention that the dynamic hrtimer softirq can lose a
>>> boosted priority. That is, if a softirq is running while a timeout
>>> happens, and the call back is of lower priority than the currently
>>> running hrtimer softirq, the timer interrupt will still lower the
>>> hrtimer softirq.
>>> Here's the problem code:
>>> static void wakeup_softirqd_prio(int softirq, int prio)
>>> {
>>> /* Interrupts are disabled: no need to stop preemption */
>>> struct task_struct *tsk = __get_cpu_var(ksoftirqd[softirq].tsk);
>>> if (tsk) {
>>> if (tsk->normal_prio != prio) {
>>> struct sched_param param;
>>> param.sched_priority = MAX_RT_PRIO-1 - prio;
>>> setscheduler(tsk, -1, SCHED_FIFO, &param);
>>> }
>>> if(tsk->state != TASK_RUNNING)
>>> wake_up_process(tsk);
>>> }
>>> }
>>> So, tsk could be softirqd-hrmono and we lower the priority. (only
>>> normal_prio is checked versus prio).
>>> So this can be a problem, if the softirq function holds a lock of a high
>>> priority task, and is running boosted. If another timer goes off with a
>>> lower priority, we can lower the priority of the softirqd and lose the
>>> inherited priority that it was running at.
>> There is a check for that inside setscheduler():
>> p->prio = rt_mutex_getprio(p);
> OK, you are right about this. The PI chain should not be affected. But
> this could still be a problem if the softirq was running at a high prio
> for a task when a lower prio callback needs to be made. It looks like
> timer is removed from the base before the function runs. So when the
> interrupt looks at the base to determine the priority to set it at, it
> might actually lower the priority of a running hrtimer thread.

That is a simple bug which ought to be simple fixable.

> -- Steve
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