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SubjectRe: [PATCH] pooling NUMA scheduler with initial load balancing
Date
> > Starting migration thread for cpu 3
> > Bringing up 4
> > CPU>dividNOWrro!
>
> I got the same thing on 2.5.40-mm1. It looks like it may be a a divide by
> zero in calc_pool_load. I am attempting to boot a band-aid version right
> now. OK, got a little further:
This opened my eyes, thanks for all your help and patience!!!

The problem is that the load balancer is called before the CPU pools
were set up. That's fine, I thought, because I define in sched_init
the default pool 0 to include all CPUs. But: in find_busiest_queue()
the cpu_to_node(this_cpu) delivers a non-zero pool which is not set up
yet, therefore pool_nr_cpus[pool]=0 and we get a zero divide.

I'm still wondering why this doesn't happen on our architecture. Maybe
the interrupts are disabled longer, I'll check. Anyway, a fix is to
force this_pool to be 0 as long as numpools=1. The attached patch is a
quick untested hack, maybe one can do it better. Has to be applied on top
of the other 2.

Going to sleep now...

Bye,
Erich
diff -urNp 2.5.39-disc-ns/kernel/sched.c 2.5.39-disc-ns8/kernel/sched.c
--- 2.5.39-disc-ns/kernel/sched.c Wed Oct 9 17:06:04 2002
+++ 2.5.39-disc-ns8/kernel/sched.c Thu Oct 10 00:51:20 2002
@@ -774,7 +774,7 @@ static inline runqueue_t *find_busiest_q
runqueue_t *busiest = NULL;
int imax, best_cpu, pool, max_pool_load, max_pool_idx;
int i, del_shift;
- int avg_load=-1, this_pool = cpu_to_node(this_cpu);
+ int avg_load=-1, this_pool;

/* Need at least ~25% imbalance to trigger balancing. */
#define BALANCED(m,t) (((m) <= 1) || (((m) - (t))/2 < (((m) + (t))/2 + 3)/4))
@@ -784,10 +784,13 @@ static inline runqueue_t *find_busiest_q
else
*nr_running = this_rq->prev_nr_running[this_cpu];

+ this_pool = (numpools == 1 ? 0 : cpu_to_node(this_cpu));
best_cpu = calc_pool_load(this_rq->load, this_cpu, this_pool, idle);

if (best_cpu != this_cpu)
goto check_out;
+ else if (numpools == 1)
+ goto out;

scan_all:
best_cpu = -1;
@@ -830,7 +833,7 @@ static inline runqueue_t *find_busiest_q
if (!BALANCED(this_rq->load[0][best_cpu],*nr_running)) {
busiest = cpu_rq(best_cpu);
this_rq->wait_node = -1;
- } else if (avg_load == -1)
+ } else if (avg_load == -1 && numpools > 1)
/* only scanned local pool, so let's look at all of them */
goto scan_all;
out:
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