Re: smpnice loadbalancing with high priority tasks

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

Peter Williams wrote:
> Siddha, Suresh B wrote:
>>>> c) DP system: if the cpu-0 has two high priority and cpu-1 has one 
>>>> normal
>>>> priority task, how can the current code detect this imbalance..
>>> How would it not?
>>
>> imbalance will be always < busiest_load_per_task and
>> max_load - this_load will be < 2 * busiest_load_per_task...
>> and pwr_move will be <= pwr_now...
> 
> I had thought about substituting (busiest_load_per_task + 
> this_load_per_task) for (busiest_load_per_task * 2) but couldn't 
> convince myself that it was the right thing to do.  (The final update to 
> this_load_per_task would need to be moved.)  The reason I couldn't 
> convince myself is that I thought it might be too aggressive and cause 
> excessive balancing.  Maybe something more sophisticated is needed to 
> prevent that possibility.  It should be noted that the relative sizes of 
> busiest_load_per_task and this_load_per_task my be useful in deciding 
> what to do in these cases.  I'll put some thought into that.

How does this bit of code look?

if (busiest_load_per_task > this_load_per_task) {
	if (max_load - this_load > busiest_load_per_task) {
		*imbalance = busiest_load_per_task;
		return busiest;
	}
} else if (max_load - this_load >= busiest_load_per_task*2) {
	*imbalance = busiest_load_per_task;
	return busiest;
}

My maths indicate this will work even in cases when the difference 
between the two load per task values is small.  By "work" I mean that it 
will one of the high priority tasks and it won't bounce back. Do you agree?

The actual patch would be a little neater than this, of course.

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

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