Re: [PATCH] sched/fair: fix load_balance redo for null imbalance

[Peter Zijlstra]

On Fri, Sep 07, 2018 at 09:51:04AM +0200, Vincent Guittot wrote:
> It can happen that load_balance finds a busiest group and then a busiest rq
> but the calculated imbalance is in fact null.

Cute. Does that happen often?

> If the calculated imbalance is null, it's useless to try to find a busiest
> rq as no task will be migrated and we can return immediately.
> 
> This situation can happen with heterogeneous system or smp system when RT
> tasks are decreasing the capacity of some CPUs.

Is it the result of one of those "force_balance" conditions in
find_busiest_group() ? Should we not fix that to then return NULL
instead?