Re: [PATCH 1/2] sched: calculate_imbalance: Fix local->avg_load >sds->avg_load case

[Vladimir Davydov]

On 09/16/2013 09:52 AM, Peter Zijlstra wrote:
> 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<vdavydov@xxxxxxxxxxxxx>
> > ---
> >   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?

Of course it does, but env->imbalance will be assigned to 0 anyway in = 
case, so why not go shortcut?
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