On Thu, Dec 13, 2018 at 12:04:20PM +0100, Vincent Guittot wrote: > On Thu, 13 Dec 2018 at 11:44, Peter Zijlstra <pet...@infradead.org> wrote: > > > > On Mon, Sep 10, 2018 at 04:43:09PM +0200, Vincent Guittot wrote: > > > When CPUs have different capacity because of RT/DL tasks or > > > micro-architecture or max frequency differences, there are situation where > > > the imbalance is not correctly set to migrate waiting task on the idle > > > CPU. > > > > > > The UC uses the force_balance case: > > > > > > if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) && > > > busiest->group_no_capacity) > > > goto force_balance; > > > > > > But calculate_imbalance fails to set the right amount of load to migrate > > > a task because of the special condition: > > > > > > busiest->avg_load <= sds->avg_load || local->avg_load >= sds->avg_load) > > > > > > Add in fix_small_imbalance, this special case that triggered the force > > > balance in order to make sure that the amount of load to migrate will be > > > enough. > > > > So I think this patch is going in the wrong direction for a number of > > reasons: > > > > - we'd like to get rid of fix_small_imbalance(), and this adds to it; > > > > - the whole load_per_task stuff is terminally broken, it _cannot_ work > > right. > > > > > > What I've suggested in the past is parameterizing the load balancer and > > picking different criteria to balance on: > > This patch is clearly a fix of the current implementation. > What you suggest below makes sense but implies a significant rework in > the calculate_imbalance and the load_balancer in general and will need > more time to reach a stable state. > Nevertheless, I will have a look at that > > I imagine that your feedback for https://lkml.org/lkml/2018/10/2/283 > will be the same ?
No; those actually look ok. It is mostly that I really don't think load_per_task makes any kind of sense. It sorta works when all tasks are of the same weight, but if you start using nice -- or way worse, cgroups -- then the number is complete bollocks.