linux-kernel - Re: [RT] rt priority losing

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Date:	Mon, 24 Jul 2006 18:54:58 +0100 (BST)
From:	Esben Nielsen <nielsen.esben@...glemail.com>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Esben Nielsen <nielsen.esben@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"Duetsch, Thomas LDE1" <thomas.duetsch@...mens.com>
Subject: Re: [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.

Esben
> -- Steve
>
>
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