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

[Peter Zijlstra]

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 at parallels.com>
> ---
>  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?