Re: -mm seems significanty slower than mainline on kernbench

[Martin Bligh <mbligh@google.com>]

>>
>> 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@bigpond.com.au>

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;
>  		}