Subject: Re: [PATCH 5/5] sched: make fix_small_imbalance work with asymmetric packing

From: Peter Zijlstra <[email-masked]>
Date: Tue, 13 Apr 2010 14:29:29 +0200

On Fri, 2010-04-09 at 16:21 +1000, Michael Neuling wrote:
> With the asymmetric packing infrastructure, fix_small_imbalance is
> causing idle higher threads to pull tasks off lower threads.  
> 
> This is being caused by an off-by-one error.  
> 
> Signed-off-by: Michael Neuling <[email-masked]>
> ---
> I'm not sure this is the right fix but without it, higher threads pull
> tasks off the lower threads, then the packing pulls it back down, etc
> etc and tasks bounce around constantly.

Would help if you expand upon the why/how it manages to get pulled up.

I can't immediately spot anything wrong with the patch, but then that
isn't my favourite piece of code either.. Suresh, any comments?

> ---
> 
>  kernel/sched_fair.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux-2.6-ozlabs/kernel/sched_fair.c
> ===================================================================
> --- linux-2.6-ozlabs.orig/kernel/sched_fair.c
> +++ linux-2.6-ozlabs/kernel/sched_fair.c
> @@ -2652,7 +2652,7 @@ static inline void fix_small_imbalance(s
>  						 * SCHED_LOAD_SCALE;
>  	scaled_busy_load_per_task /= sds->busiest->cpu_power;
>  
> -	if (sds->max_load - sds->this_load + scaled_busy_load_per_task >=
> +	if (sds->max_load - sds->this_load + scaled_busy_load_per_task >
>  			(scaled_busy_load_per_task * imbn)) {
>  		*imbalance = sds->busiest_load_per_task;
>  		return;