typecast bug in sched.c bites reschedule_idle on alpha

[Andy Isaacson <adi@hexapodia.org>]

On SMP Alpha, a saturated system (with one busy userland process per
CPU) becomes almost completely unusable.  The busy loops don't have to
be doing anything more complicated than "while(1) ;".  The symptom is
that even logging into the system (with ssh) takes on the order of 30
seconds.  (Tested on an ES40 with 4 processors, 2.4.18.)

It turns out that the problem is in kernel/schedule.c:reschedule_idle.

        cycles_t oldest_idle;
...
        oldest_idle = (cycles_t) -1;
...
                        if (oldest_idle == -1ULL) {

Since asm-alpha/timex.h defines cycles_t as unsigned int, this
comparison is always false.  Changing it to (cycles_t)-1 fixes the
problem.

Patch below.  I'd like this to go into 2.4.22, if nobody has a problem
with that.  I would like confirmation from other eyes -- this patch
doesn't break semantics on any architecture, does it?

-andy

--- linux-2.4.21/kernel/sched.c	Thu Jul 17 16:43:37 2003
+++ linux-2.4.21-idle-fix/kernel/sched.c	Thu Jul 17 16:23:25 2003
@@ -282,7 +282,7 @@
 				target_tsk = tsk;
 			}
 		} else {
-			if (oldest_idle == -1ULL) {
+			if (oldest_idle == (cycles_t)-1) {
 				int prio = preemption_goodness(tsk, p, cpu);
 
 				if (prio > max_prio) {
@@ -294,7 +294,7 @@
 	}
 	tsk = target_tsk;
 	if (tsk) {
-		if (oldest_idle != -1ULL) {
+		if (oldest_idle != (cycles_t)-1) {
 			best_cpu = tsk->processor;
 			goto send_now_idle;
 		}