rt priority losing

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.

-- Steve

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Quoted 31 lines · Steven Rostedt

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).

Quoted 4 lines

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);

Quoted 7 lines

Quoted 4 lines

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.

Quoted 33 lines · Esben Nielsen

Quoted 31 lines · Steven Rostedt

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).

Quoted 7 lines

Quoted 4 lines

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.

-- Steve

Quoted 8 lines

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.

I guess the simple fix is not to allow the interrupt to lower the priority. But simple fixes do not always handle all the cases.

Thomas G., see any side effects with this patch?

-- Steve

Signed-off-by Steven Rostedt <[email protected]>