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: - nr_running ; if there are idle CPUs around - utilization ; if there's idle time (u<1) - weight And I suppose you're hitting one of the nr_running cases here. > Signed-off-by: Vincent Guittot <vincent.guit...@linaro.org> > --- > kernel/sched/fair.c | 14 ++++++++++++++ > 1 file changed, 14 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 309c93f..72bc5e8 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -8048,6 +8048,20 @@ void fix_small_imbalance(struct lb_env *env, struct > sd_lb_stats *sds) > local = &sds->local_stat; > busiest = &sds->busiest_stat; > > + /* > + * There is available capacity in local group and busiest group is > + * overloaded but calculate_imbalance can't compute the amount of load > + * to migrate because load_avg became meaningless due to asymetric > + * capacity between groups. In such case, we only want to migrate at > + * least one tasks of the busiest group and rely of the average load > + * per task to ensure the migration. > + */ > + if (env->idle != CPU_NOT_IDLE && group_has_capacity(env, local) && > + busiest->group_no_capacity) { > + env->imbalance = busiest->load_per_task; > + return; > + } > + > if (!local->sum_nr_running) > local->load_per_task = cpu_avg_load_per_task(env->dst_cpu); > else if (busiest->load_per_task > local->load_per_task) > -- > 2.7.4 >