Re: [PATCH 4/4] sched: bias to target cpu load to reduce task moving

[Vincent Guittot]

On 7 January 2014 14:15, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, Jan 07, 2014 at 01:59:30PM +0100, Peter Zijlstra wrote:
>> On Tue, Jan 07, 2014 at 12:55:18PM +0000, Morten Rasmussen wrote:
>> > My understanding is that should_we_balance() decides which cpu is
>> > eligible for doing the load balancing for a given domain (and the
>> > domains above). That is, only one cpu in a group is allowed to load
>> > balance between the local group and other groups. That cpu would
>> > therefore be reponsible for pulling enough load that the groups are
>> > balanced even if it means temporarily overloading itself. The other cpus
>> > in the group will take care of load balancing the extra load within the
>> > local group later.
>>
>> Correct.
>
> On that; one of the things I wanted to (and previously did attempt but
> failed) is trying to rotate this cpu. Currently its always the first cpu
> (of the group) and that gives a noticeable bias.

Isn't the current policy (it's the 1st idle cpu in priority). a good
enough way to rotate the cpus ? Are you need the rotation for loaded
use case too ?

>
> If we could slowly rotate the cpu that does this that would alleviate
> both the load and cost bias.
>
> One thing I was thinking of is keeping a global counter maybe:
>  'x := jiffies >> n'
> might be good enough and using the 'x % nr_cpus_in_group'-th cpu
> instead.
>
> Then again, these are micro issue and not a lot of people complain
> about this.
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