[PATCH] SCHED_FIFO and SCHED_RR scheduler fix, kernel 2.2.21

From: Bhavesh P. Davda (bhavesh@avaya.com)
Date: Fri Jun 21 2002 - 17:12:56 EST

The 2.2.21 kernel was behaving incorrectly for SCHED_FIFO and SCHED_RR
scheduling.

The correct behaviour for SCHED_FIFO is priority preemption: run to
completion, or system call, or preemption by higher priority process. The
correct behaviour for SCHED_RR is the same as SCHED_FIFO for the
preemption case, or run for a time slice, and go to the back of the run
queue for that priority.

More details can be found at:

http://www.opengroup.org/onlinepubs/7908799/xsh/realtime.html

This is a small patch, but fixes the behaviour for SCHED_FIFO and SCHED_RR
scheduling in the 2.2.21 kernel. It also improves the efficiency of the
kernel by NOT calling schedule() for every tick for a SCHED_FIFO process. 

--
Bhavesh P. Davda
Avaya, Inc.
bhavesh@avaya.com

diff -aur linux-2.2.21/kernel/sched.c linux-2.2.21-bpd/kernel/sched.c
--- linux-2.2.21/kernel/sched.c	Sun Mar 25 09:37:40 2001
+++ linux-2.2.21-bpd/kernel/sched.c	Fri Jun 21 09:54:55 2002
@@ -749,7 +749,16 @@
 	/* Default process to select.. */
 	next = idle_task(this_cpu);
 	c = -1000;
-	if (prev->state == TASK_RUNNING)
+	/* 
+	 * If a SCHED_RR task has exhausted its time slice, 
+	 * it is at the back of the runqueue, even if it is
+	 * still running. On the other hand, if a SCHED_RR task
+	 * is still running and still has time left in its
+	 * time slice, then it is still the first process in its
+	 * priority band, so it will run next if it is the first
+	 * highest priority SCHED_RR task
+	 */
+	if ((prev->state == TASK_RUNNING) && (prev->policy != SCHED_RR))
 		goto still_running;
 still_running_back:
 
@@ -1492,7 +1501,12 @@
 		p->counter -= ticks;
 		if (p->counter < 0) {
 			p->counter = 0;
-			p->need_resched = 1;
+			/* SCHED_FIFO is priority preemption, so this is
+			 * not the place to reschedule it
+			 */
+			if (p->policy != SCHED_FIFO) {
+				p->need_resched = 1;
+			}
 		}
 		if (p->priority < DEF_PRIORITY)
 			kstat.cpu_nice += user;
@@ -1785,8 +1799,6 @@
 	retval = 0;
 	p->policy = policy;
 	p->rt_priority = lp.sched_priority;
-	if (p->next_run)
-		move_first_runqueue(p);
 
 	current->need_resched = 1;

-
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This archive was generated by hypermail 2b29 : Sun Jun 23 2002 - 22:00:25 EST