Re: -mm seems significanty slower than mainline on kernbench

[Peter Williams <pwil3058@bigpond.net.au>]

Martin Bligh wrote:
> 
>>>
>>> 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.

OK, thanks.  I think there's a few more places where SCHED_LOAD_SCALE 
needs to be multiplied by NICE_TO_BIAS_PRIO(0).  Basically, anywhere 
that it's added to, subtracted from or compared to a load.  In those 
cases it's being used as a scaled version of 1 and we need a scaled 
version of NICE_TO_BIAS_PRIO(0).  I'll have another patch later today.

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce