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SubjectRe: [Question] Some question about Ingo scheduler.
Steven Rostedt Wrote:

>[added back the LKML since others might learn from this too]
>On Tue, 11 Oct 2005, liyu wrote:
>>For first question, I have a few confused yet.
>>As Steven said, if PREEMPT_ACTIVE is set, we cann't preempt current task,
>>because of
>>we can not wake up it later, however, the process of task switch will
>>save information
>>of the task context to task_struct (also thread_info), why we cann't wake
>>up it?
>First let me corrent that statement. I said that if PREEMPT_ACTIVE is
>set, we can't take the task off the run queue. I didn't say we can't
>preempt that task, since that _is_ what is about to happen.
>OK I worded it wrong. I shouldn't say we "can't" wake it up. What I
>should have said is that we may not know to wake it up. You are right,
>all the information is there to wake it up but the case might happen where
>we just don't know to do it.
>Here's some common code to look at.
>add_wait_queue(q, wait);
>if (!some_event)
> schedule();
>remove_wait_queue(q, wait);
>This above code isn't directly from the kernel but the logic of it is all
>over the place. So the task is going to wait for some event, and when
>that event happens, it will wake up all the tasks that are on the wait
>queue. Now what happens if the event happened before the
>set_current_state(TASK_UNINTERRUPTIBLE)? Normally that would be OK
>because of the check to see if some_event happened, and if it did then
>don't call schedule.
>Now back to that PREEMPT_ACTIVE check. If the above case happens, and then
>the task is preempted before it set itself back to TASK_RUNNING, without
>the PREEMPT_ACTIVE check in schedule, the process would be removed from
>the task run queue. That means it is no longer in the queue to be
>scheduled. But the event already happened that would have woken it back
>up. So this task would forever stay in the TASK_UNINTERRUPTIBLE state and
>never wake up. The PREEMPT_ACTIVE check is to allow the task to stay on
>the run queue until it gets to a point that itself calls schedule. As the
>above logic might allow (if the event has not happened yet).
>So what determines what can be scheduled, is the fact that the task is on
>the run queue, _not_ whether or not the task is in the TASK_RUNNING state.
>At least with preemption enabled. Not being in TASK_RUNNING will take the
>task off the run queue when that task calls schedule itself, not when it
>is preempted.
>Does this make more sense?
>-- Steve
Hi, Steve:

Thanks for so detailed explain.

It seem I am not understand what is sleep and wakeup truly.

What's your mean of "in runqueue"? I think you mean the
task_struct is in one priority array (active or expired)
of one queue. the schedule() only can process task in runqueue.
In deactivate_task(), it will reset task_struct->array to NULL,
After call it, we can not wake up that task.

However, I read try_to_wake_up(), and found it can handle that case
which task_struct->array is NULL, it will be call activate_task()
to insert task to one runqueue. and default_wake_function() will
call try_to_wake_up(), so we still can wake up it.

I am confused again. this quesion is more interesting and more.

Wait for reply.

Good luck.


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