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SubjectRe: [PATCH v3 04/10] sched/fair: rework load_balance
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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).

[...]

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