Re: tvtime and the Linux 2.6 scheduler

[Ingo Molnar <mingo@elte.hu>]

* Billy Biggs <vektor@dumbterm.net> wrote:

>   for(;;)
>       9 ms : process frame
>       4 ms : draw frame
>       3 ms : wait until next field time using /dev/rtc
>       9 ms : process frame
>       4 ms : draw frame
>       3 ms : block on /dev/video0 for next frame
>      -----
>      33 ms : time per NTSC frame
> 
>   The theory is that Linux classifies this as a CPU hog regardless of
> its priority, and preempts tvtime with other processes. [...]

this would indicate a pretty broken scheduler. To prove (or exclude)
this possibility, could you apply the attached debugging patch? The
patch checks whether we ever switch away from a still running (and
hence, on-runqueue) RT task to a non-RT task.

(NOTE: dont run this patch on an SMP kernel, the debugging message will
deadlock there due to spinlock recursion.)

	Ingo

--- linux/kernel/sched.c.orig	
+++ linux/kernel/sched.c	
@@ -2279,6 +2279,7 @@ switch_tasks:
 	prev->timestamp = now;
 
 	if (likely(prev != next)) {
+		WARN_ON(rt_task(prev) && (prev->state == TASK_RUNNING) && !rt_task(next));
 		next->timestamp = now;
 		rq->nr_switches++;
 		rq->curr = next;