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Subject[patch 15/29] Fix divide-by-zero in the 2.6.23 scheduler code

2.6.23-stable review patch. If anyone has any objections, please let us
know.

------------------
From: Chuck Ebbert <cebbert@redhat.com>

No patch in mainline as this logic has been removed from 2.6.24 so it is
not necessary.


https://bugzilla.redhat.com/show_bug.cgi?id=340161

The problem code has been removed in 2.6.24. The below patch disables
SCHED_FEAT_PRECISE_CPU_LOAD which causes the offending code to be skipped
but does not prevent the user from enabling it.

The divide-by-zero is here in kernel/sched.c:

static void update_cpu_load(struct rq *this_rq)
{
u64 fair_delta64, exec_delta64, idle_delta64, sample_interval64, tmp64;
unsigned long total_load = this_rq->ls.load.weight;
unsigned long this_load = total_load;
struct load_stat *ls = &this_rq->ls;
int i, scale;

this_rq->nr_load_updates++;
if (unlikely(!(sysctl_sched_features & SCHED_FEAT_PRECISE_CPU_LOAD)))
goto do_avg;

/* Update delta_fair/delta_exec fields first */
update_curr_load(this_rq);

fair_delta64 = ls->delta_fair + 1;
ls->delta_fair = 0;

exec_delta64 = ls->delta_exec + 1;
ls->delta_exec = 0;

sample_interval64 = this_rq->clock - ls->load_update_last;
ls->load_update_last = this_rq->clock;

if ((s64)sample_interval64 < (s64)TICK_NSEC)
sample_interval64 = TICK_NSEC;

if (exec_delta64 > sample_interval64)
exec_delta64 = sample_interval64;

idle_delta64 = sample_interval64 - exec_delta64;

======> tmp64 = div64_64(SCHED_LOAD_SCALE * exec_delta64, fair_delta64);
tmp64 = div64_64(tmp64 * exec_delta64, sample_interval64);

this_load = (unsigned long)tmp64;

do_avg:

/* Update our load: */
for (i = 0, scale = 1; i < CPU_LOAD_IDX_MAX; i++, scale += scale) {
unsigned long old_load, new_load;

/* scale is effectively 1 << i now, and >> i divides by scale */

old_load = this_rq->cpu_load[i];
new_load = this_load;

this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i;
}
}

For stable only; the code has been removed in 2.6.24.

Signed-off-by: Chuck Ebbert <cebbert@redhat.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/sched_fair.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -93,7 +93,7 @@ unsigned int sysctl_sched_features __rea
SCHED_FEAT_FAIR_SLEEPERS *1 |
SCHED_FEAT_SLEEPER_AVG *0 |
SCHED_FEAT_SLEEPER_LOAD_AVG *1 |
- SCHED_FEAT_PRECISE_CPU_LOAD *1 |
+ SCHED_FEAT_PRECISE_CPU_LOAD *0 |
SCHED_FEAT_START_DEBIT *1 |
SCHED_FEAT_SKIP_INITIAL *0;

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