typecast bug in sched.c bites reschedule_idle on alpha

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

@@ -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;
 		}

Nice catch, Andy. Thanks.

Ivan.

Wasn't me, it was Bruce Keller (but I don't know if he wants his email address plastered all over linux-kernel). I just cleaned up and prepared a patch with bug description.

-andy