Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair()

[Michael Wang]

On 02/21/2013 04:10 PM, Mike Galbraith wrote:
> On Thu, 2013-02-21 at 15:00 +0800, Michael Wang wrote: 
>> On 02/21/2013 02:11 PM, Mike Galbraith wrote:
>>> On Thu, 2013-02-21 at 12:51 +0800, Michael Wang wrote: 
>>>> On 02/20/2013 06:49 PM, Ingo Molnar wrote:
>>>> [snip]
>> [snip]
>>>>
>>>> 	if wake_affine()
>>>> 		new_cpu = select_idle_sibling(curr_cpu)
>>>> 	else
>>>> 		new_cpu = select_idle_sibling(prev_cpu)
>>>>
>>>> 	return new_cpu
>>>>
>>>> Actually that doesn't make sense.
>>>>
>>>> I think wake_affine() is trying to check whether move a task from
>>>> prev_cpu to curr_cpu will break the balance in affine_sd or not, but why
>>>> won't break balance means curr_cpu is better than prev_cpu for searching
>>>> the idle cpu?
>>>
>>> You could argue that it's impossible to break balance by moving any task
>>> to any idle cpu, but that would mean bouncing tasks cross node on every
>>> wakeup is fine, which it isn't.
>>
>> I don't get it... could you please give me more detail on how
>> wake_affine() related with bouncing?
> 
> If we didn't ever ask if it's ok, we'd always pull, and stack load up on
> one package if there's the tiniest of holes to stuff a task into,
> periodic balance forcibly rips buddies back apart, repeat.  At least
> with wake_affine() in the loop, there's somewhat of a damper. 

Oh, I think I got the reason why old logical check the affine_sd
firstly, so when prev_cpu and curr_cpu belong to different package,
there will be a chance to enter balance path, that seems like a solution
for this problem, amazing ;-)

You are right, with out this logical, chances to balance load between
packages will missed, I will apply it in next version.

Regards,
Michael Wang

> 
>>>> So the new logical in this patch set is:
>>>>
>>>> 	new_cpu = select_idle_sibling(prev_cpu)
>>>> 	if idle_cpu(new_cpu)
>>>> 		return new_cpu
>>>
>>> So you tilted the scales in favor of leaving tasks in their current
>>> package, which should benefit large footprint tasks, but should also
>>> penalize light communicating tasks.
>>
>> Yes, I'd prefer to wakeup the task on a cpu which:
>> 1. idle
>> 2. close to prev_cpu
>>
>> So if both curr_cpu and prev_cpu have idle cpu in their topology, which
>> one is better? that depends on how task benefit from cache and the
>> balance situation, whatever, I don't think the benefit worth the high
>> cost of wake_affine() in most cases...
> 
> We've always used wake_affine() before, yet been able to schedule at
> high frequency, so I don't see that it can be _that_ expensive.  I
> haven't actually measured lately (loooong time) though.
> 
> WRT cost/benefit of migration, yeah, it depends entirely on the tasks,
> some will gain, some will lose.  On a modern single processor box, it
> just doesn't matter, there's only one llc (two s_i_s() calls = oopsie),
> but on my beloved old Q6600 or a big box, it'll matter a lot to
> something.  NUMA balance will deal with big boxen, my trusty old Q6600
> will likely get all upset with some localhost stuff.
> 
> -Mike
> 
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