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Date: Mon, 24 Jul 2006 18:00:24 +0100 (BST)
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To: Steven Rostedt <rostedt@goodmis.org>
cc: Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
       LKML <linux-kernel@vger.kernel.org>,
       "Duetsch, Thomas LDE1" <thomas.duetsch@siemens.com>
Subject: Re: [RT] rt priority losing
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From: Esben Nielsen <nielsen.esben@googlemail.com>
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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);

Esben

>
> -- Steve
>
> -
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