Re: General question about TASK_INTERRUPTIBLE and schedule_timeout()
From: Shan Hai
Date: Fri Sep 02 2011 - 04:09:09 EST
On 09/02/2011 03:31 PM, kautuk.c @samsung.com wrote:
On Fri, Sep 2, 2011 at 12:36 PM, Shan Hai<haishan.bai@xxxxxxxxx> wrote:
On 09/02/2011 02:18 PM, Shan Hai wrote:
On 09/01/2011 10:09 AM, Yong Zhang wrote:
On Wed, Aug 31, 2011 at 06:18:19PM +0530, sifram rajas wrote:
Hi,
I have a general question about the following 2 lines of code I see
all over the kernel:
1 set_current_state(TASK_INTERRUPTIBLE) ;
2 schedule_timeout(<some value>);
In the above code, if we encounter an interrupt after executing line
1, we will end up
call schedule() from the architecture specific code for CONFIG_PREEMPT
kernels, after
the interrupt handler has been invokled.
Yes.
This will cause the current task to sleep interruptibly forever
Actually, sleeping forever in the TASK_INTERRUPTIBLE state is not correct,
because even though the task is preempted by higher priority one
it will finally get a chance to run, but you will get time out value
of<some value> + preemption latency.
instead of for a certain timeout interval.
No.
schedule() will not put an preempted task to sleep, see:
This might be problematic, because on the IRQ to preemption check path
the PREEMPT_ACTIVE was already set and the following 'if' statement
could not hold because of
!(preempt_count()& PREEMPT_ACTIVE) == false
Yes.
and the pick_next_task() might put the preempted task to sleep.
pick_next_task() will simply select the next task for scheduling.
After all running tasks have been scheduled then this preempted task
will also be rescheduled
as it is still on this runqueue.
I do not think that this will put this preempted it to sleep.
Reason: Although the state is still set to TASK_INTERRUPTIBLE, the
task is not removed
from the runqueue as !(preempt_count()& PREEMPT_ACTIVE) == false.
Yep, that's right, call 'sleep' here is improper, the right way
to put it should be 'the current task is deprived of its CPU' OR
'the current lost the CPU', we call sleep when a task is
removed from the run queue anyway :-)
I mean when the state of task is TASK_INTERRUPTIBLE the preempted task will
be put to sleep, its true in sifram's case.
I disagree.Yong is still right in this scenario as the task will
remain on the runqueue due to
the !(preempt_count()& PREEMPT_ACTIVE) == false.
As stated above.
Cheers
Shan Hai
Yong is right on stating "schedule() will not put an preempted task to
sleep",
its true for the task state of which is TASK_RUNNING.
Since TASK_RUNNING is defined as 0, the task remains on the runqueue here also.
Cheers
Shan Hai
Correct me on any misunderstanding :-)
Cheers
Shan Hai
asmlinkage void __sched schduule(void)
{
...
if (prev->state&& !(preempt_count()& PREEMPT_ACTIVE)) {
if (unlikely(signal_pending_state(prev->state, prev))) {
prev->state = TASK_RUNNING;
} else {
...
}
}
...
}
Thanks,
Yong
Won't this defeat the purpose of the above code to schedule out or
sleep for a certain finite timeout ?
If yes, then what are the techniques to solve this problem ?
Thanks,
Sifram.
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