[PATCH v3 09/12] Revert "sched: Put rq's sched_avg under CONFIG_FAIR_GROUP_SCHED"

On 11 July 2014 18:13, Morten Rasmussen [off-list ref] wrote:

[snip]

In this example using rq->avg leads to imbalance whereas unweighted load
would not. Correct me if I missed anything.

You just miss to take into account how the imbalance is computed

Coming back to the previous example. I'm not convinced that inflation of
the unweighted load sum when tasks overlap in time is a bad thing. I
have mentioned this before. The average cpu utilization over the 40ms
period is 50%. However the true compute capacity demand is 200% for the
first 15ms of the period, 100% for the next 5ms and 0% for the remaining
25ms. The cpu is actually overloaded for 15ms every 40ms. This fact is
factored into the unweighted load whereas rq->avg would give you the
same utilization no matter if the tasks are overlapped or not. Hence
unweighted load would give us an indication that the mix of tasks isn't
optimal even if the cpu has spare cycles.

If you don't care about overlap and latency, the unweighted sum of task
running time (that Peter has proposed a number of times) is better
metric, IMHO. As long the cpu isn't fully utilized.

Morten