Re: busted CFS group load balancer?

[Ken Chen <kenchen@google.com>]

On Mon, Nov 17, 2008 at 9:19 PM, Peter Zijlstra wrote:
> Note that with larger cpu count and/or lower group weight we'll quickly
> run into numerical trouble...
>
> I would recommend trying this with the minimum weight in the order of
> 8-16 times number of cpus on your system.
>
> There is only so much one can do with 10 bit fixed precision math :/

That is probably one of the many problems.  I also found that the
updates to the per-cpu task_group's sched_entity load weight
(tg->se[cpu]->load.weight) is very problematic and very erratic.

The total rq_weight is calculated at one beginning of tg_shares_up(),

        for_each_cpu_mask(i, sd->span) {
                rq_weight += tg->cfs_rq[i]->load.weight;
                shares += tg->cfs_rq[i]->shares;
        }

However, the scaling of per-cpu se->load.weight in function
__update_group_shares_cpu() takes another lookup of
tg->cfs_rq[cpu]->load.weight at a different time.
cfs_rq[cpu].load.weight aren't always consistent across these two
times.  Due to these inconsistency of value taken on per cpu cfs_rq,
I've see tg->se[cpu]->load.weight jumping all over the place.  In our
environment, the cpu loads are very dynamic.  Process
queuing/dequeuing at high rate.

I'm also very troubled with this calculation in __update_group_shares_cpu():

        shares = (sd_shares * rq_weight) / (sd_rq_weight + 1);

Won't you have rounding problem here?  value 'shares' will gradually
decrease for each iteration of __update_group_shares_cpu()?

- Ken