On 19/09/2019 08:33, Vincent Guittot wrote: [...]
> @@ -8283,69 +8363,133 @@ static inline void update_sd_lb_stats(struct lb_env > *env, struct sd_lb_stats *sd > */ > static inline void calculate_imbalance(struct lb_env *env, struct > sd_lb_stats *sds) > { > - unsigned long max_pull, load_above_capacity = ~0UL; > struct sg_lb_stats *local, *busiest; > > local = &sds->local_stat; > busiest = &sds->busiest_stat; > > - if (busiest->group_asym_packing) { > + if (busiest->group_type == group_misfit_task) { > + /* Set imbalance to allow misfit task to be balanced. */ > + env->balance_type = migrate_misfit; > + env->imbalance = busiest->group_misfit_task_load; > + return; > + } > + > + if (busiest->group_type == group_asym_packing) { > + /* > + * In case of asym capacity, we will try to migrate all load to > + * the preferred CPU. > + */ > + env->balance_type = migrate_load; > env->imbalance = busiest->group_load; > return; > } > > + if (busiest->group_type == group_imbalanced) { > + /* > + * In the group_imb case we cannot rely on group-wide averages > + * to ensure CPU-load equilibrium, try to move any task to fix > + * the imbalance. The next load balance will take care of > + * balancing back the system. > + */ > + env->balance_type = migrate_task; > + env->imbalance = 1; > + return; > + } > + > /* > - * Avg load of busiest sg can be less and avg load of local sg can > - * be greater than avg load across all sgs of sd because avg load > - * factors in sg capacity and sgs with smaller group_type are > - * skipped when updating the busiest sg: > + * Try to use spare capacity of local group without overloading it or > + * emptying busiest > */ > - if (busiest->group_type != group_misfit_task && > - (busiest->avg_load <= sds->avg_load || > - local->avg_load >= sds->avg_load)) { > - env->imbalance = 0; > + if (local->group_type == group_has_spare) { > + if (busiest->group_type > group_fully_busy) { > + /* > + * If busiest is overloaded, try to fill spare > + * capacity. This might end up creating spare capacity > + * in busiest or busiest still being overloaded but > + * there is no simple way to directly compute the > + * amount of load to migrate in order to balance the > + * system. > + */ > + env->balance_type = migrate_util; > + env->imbalance = max(local->group_capacity, > local->group_util) - > + local->group_util; > + return; > + } > + > + if (busiest->group_weight == 1 || sds->prefer_sibling) { > + /* > + * When prefer sibling, evenly spread running tasks on > + * groups. > + */ > + env->balance_type = migrate_task; > + env->imbalance = (busiest->sum_h_nr_running - > local->sum_h_nr_running) >> 1; Isn't that one somewhat risky? Say both groups are classified group_has_spare and we do prefer_sibling. We'd select busiest as the one with the maximum number of busy CPUs, but it could be so that busiest.sum_h_nr_running < local.sum_h_nr_running (because pinned tasks or wakeup failed to properly spread stuff). The thing should be unsigned so at least we save ourselves from right shifting a negative value, but we still end up with a gygornous imbalance (which we then store into env.imbalance which *is* signed... Urgh). [...]