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

[Peter Zijlstra <peterz@infradead.org>]

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.

> I may have missed something, but I don't understand the reason for the
> performance improvements that you are reporting. I see better numbers
> for a few benchmarks, but I still don't understand why the code makes
> sense after the cleanup. If we don't understand why it works, we cannot
> be sure that it doesn't harm other benchmarks. There is always a chance
> that we miss something but, IMHO, not having any idea to begin with
> increases the chances for problems later significantly. So why not get
> to the bottom of the problem of cleaning up cpu_load?
> 
> Have you done more extensive benchmarking? Have you seen any regressions
> in other benchmarks?

I only remember hackbench numbers and that generally fares well with a
more aggressive balancer since it has no actual work to speak of the
migration penalty is very low and because there's a metric ton of tasks
the aggressive leveling makes for more coherent 'throughput'.