Scheduling bug for SCHED_FIFO and SCHED_RR

[Nigel Gamble]

A SCHED_FIFO or SCHED_RR task with priority n+1 will not preempt a
running task with priority n.  You need to give the higher priority task
a priority of at least n+2 for it to be chosen by the scheduler.

The problem is caused by reschedule_idle(), uniprocessor version:

	if (preemption_goodness(tsk, p, this_cpu) > 1)
		tsk->need_resched = 1;

For real-time scheduling to work correctly, need_resched should be set
whenever preemption_goodness() is greater than 0, not 1.

Here is a patch against 2.4.3:

--- 2.4.3/kernel/sched.c	Thu Apr 19 15:03:21 2001
+++ linux/kernel/sched.c	Fri Apr 20 16:45:07 2001
@@ -290,7 +290,7 @@
 	struct task_struct *tsk;
 
 	tsk = cpu_curr(this_cpu);
-	if (preemption_goodness(tsk, p, this_cpu) > 1)
+	if (preemption_goodness(tsk, p, this_cpu) > 0)
 		tsk->need_resched = 1;
 #endif
 }

Nigel Gamble                                    [e-mail]
Mountain View, CA, USA.                         http://www.nrg.org/

MontaVista Software                             [e-mail]