Re: [patch] Real-Time Preemption, deactivate() scheduling issue

[Ingo Molnar]

* Eugeny S. Mints <emints@xxxxxxxxxxxxx> wrote:

> please consider the following scenario for full RT kernel.
> 
> Task A is running then an irq is occured which in turn wakes up irq 
> related thread (B) of a higher priority than A.
> 
> my current understanding that actual context switch between A and B will 
> occure at preempt_schedule_irq() on the "return form irq " path.
> 
> in this case the following "if" statement in __schedule() always returns 
> false since  preempt_schedule_irq() always sets up  PREEMPT_ACTIVE 
> before __schedule() call.
> 
>         if ((prev->state & ~TASK_RUNNING_MUTEX) &&
>                         !(preempt_count() & PREEMPT_ACTIVE)) {
> 
> as result the deactivate() is never called for preempted task A in this 
> scenario. BUt if the task A is preempted while not in TASK_RUNNING state 
> such behaviour seems incorrect since we get a task in not TASK_RUNNING 
> state linked into a run queue.

this behavior is intentional: 'forced preemption' (of any sort, even in 
the upstream kernel's CONFIG_PREEMPT model) should not impact the task's 
state. So it does not modify p->state. [ The TASK_RUNNING_MUTEX state 
furthermore enables wakeups to occur in an invariant way: even though 
technically the tasks are on the runqueue, a 'normal' wakeup is still 
noticed and later on acted upon.]

this is very important for forced preemption to not impact the coding 
model of kernel code that is normally tested with !PREEMPT. (the 
TASK_RUNNING_MUTEX scheduler feature furthermore enables us to preempt 
without impacting wakeup logic.)

> An example:
> 
> drivers/net/irda/sir_dev.c: 76 (2.6.10 kernel)
> 
>         spin_lock_irqsave(&dev->tx_lock, flags); /* serialize th other 
> tx operations */
>         while (dev->tx_buff.len > 0) {    /* wait until tx idle */
>                 spin_unlock_irqrestore(&dev->tx_lock, flags);
> 76:             set_current_state(TASK_UNINTERRUPTIBLE);
>                 schedule_timeout(msecs_to_jiffies(10));
>                 spin_lock_irqsave(&dev->tx_lock, flags);
>         }
> 
> At  line 76 irqs are enabled, preemption is enabled.
> Let assume the task A executes this code and gets preempted right after 
> line 76. Task state is TASK_UNINTERRUPTIBLE but it will not be 
> deactevated. Of cource this is the bug in set_current_state() 
> utilization in this particular driver but schedule stuff should be 
> robust to such bugs I believe. There are a lot such bugs in the kernel I 
> believe.

it is not a problem to have tasks with TASK_UNINTERRUPTIBLE on the 
runqueue - this happens every day with CONFIG_PREEMPT kernels, and it's 
fully intentional. Can you see any bugs caused by this behavior?

	Ingo
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