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

From: Peter Zijlstra
Date: Tue Jan 07 2014 - 08:16:30 EST

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.

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

Then again, these are micro issue and not a lot of people complain
about this.
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