Re: -mm seems significanty slower than mainline on kernbench

[Martin Bligh]

> > But I was thinking more about the code that (in the original) handled 
> > the case where the number of tasks to be moved was less than 1 but 
> > more than 0 (i.e. the cases where "imbalance" would have been reduced 
> > to zero when divided by SCHED_LOAD_SCALE).  I think that I got that 
> > part wrong and you can end up with a bias load to be moved which is 
> > less than any of the bias_prio values for any queued tasks (in 
> > circumstances where the original code would have rounded up to 1 and 
> > caused a move).  I think that the way to handle this problem is to 
> > replace 1 with "average bias prio" within that logic.  This would 
> > guarantee at least one task with a bias_prio small enough to be moved.
> >
> > I think that this analysis is a strong argument for my original patch 
> > being the cause of the problem so I'll go ahead and generate a fix. 
> > I'll try to have a patch available later this morning.
>
>
> Attached is a patch that addresses this problem.  Unlike the description 
> above it does not use "average bias prio" as that solution would be very 
> complicated.  Instead it makes the assumption that NICE_TO_BIAS_PRIO(0) 
> is a "good enough" for this purpose as this is highly likely to be the 
> median bias prio and the median is probably better for this purpose than 
> the average.
>
> Signed-off-by: Peter Williams <pwil3058@xxxxxxxxxxxxxx>

Doesn't fix the perf issue.

M.


> Peter
>
>
> ------------------------------------------------------------------------
>
> Index: MM-2.6.X/kernel/sched.c
> ===================================================================
> --- MM-2.6.X.orig/kernel/sched.c	2006-01-12 09:23:38.000000000 +1100
> +++ MM-2.6.X/kernel/sched.c	2006-01-12 10:44:50.000000000 +1100
> @@ -2116,11 +2116,11 @@ find_busiest_group(struct sched_domain *
>  				(avg_load - this_load) * this->cpu_power)
>  			/ SCHED_LOAD_SCALE;
>  
> -	if (*imbalance < SCHED_LOAD_SCALE) {
> +	if (*imbalance < NICE_TO_BIAS_PRIO(0) * SCHED_LOAD_SCALE) {
>  		unsigned long pwr_now = 0, pwr_move = 0;
>  		unsigned long tmp;
>  
> -		if (max_load - this_load >= SCHED_LOAD_SCALE*2) {
> +		if (max_load - this_load >= NICE_TO_BIAS_PRIO(0) * SCHED_LOAD_SCALE*2) {
>  			*imbalance = NICE_TO_BIAS_PRIO(0);
>  			return busiest;
>  		}

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