On Sun, Sep 15, 2013 at 05:49:13PM +0400, Vladimir Davydov wrote: > In busiest->group_imb case we can come to calculate_imbalance() with > local->avg_load >= busiest->avg_load >= sds->avg_load. This can result > in imbalance overflow, because it is calculated as follows > > env->imbalance = min( > max_pull * busiest->group_power, > (sds->avg_load - local->avg_load) * local->group_power > ) / SCHED_POWER_SCALE; > > As a result we can end up constantly bouncing tasks from one cpu to > another if there are pinned tasks. > > Fix this by skipping the assignment and assuming imbalance=0 in case > local->avg_load > sds->avg_load. > -- > The bug can be caught by running 2*N cpuhogs pinned to two logical cpus > belonging to different cores on an HT-enabled machine with N logical > cpus: just look at se.nr_migrations growth. > > Signed-off-by: Vladimir Davydov <[email protected]> > --- > kernel/sched/fair.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 9b3fe1c..507a8a9 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4896,7 +4896,8 @@ static inline void calculate_imbalance(struct lb_env > *env, struct sd_lb_stats *s > * max load less than avg load(as we skip the groups at or below > * its cpu_power, while calculating max_load..) > */ > - if (busiest->avg_load < sds->avg_load) { > + if (busiest->avg_load <= sds->avg_load || > + local->avg_load >= sds->avg_load) { > env->imbalance = 0; > return fix_small_imbalance(env, sds); > }
Why the = part? Surely 'busiest->avg_load < sds->avg_load || local->avg_load > sds->avg_load' avoids both underflows? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
