Re: [RT] rt priority losing

[Esben Nielsen]

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.

Esben
> -- Steve
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/