Re: [PATCH 2/6] sched/fair: Check whether active load balance is needed in busiest group

[Vincent Guittot]

On Thu, 4 May 2023 at 18:11, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:
>
> From: Tim C Chen <tim.c.chen@xxxxxxxxxxxxxxx>
>
> In the busiest group, we need to consider whether active load balance
> to a local group is needed even when it is not overloaded.  For example,
> when the busiest group is a SMT group that's fully busy and the destination group
> is a cluster group with idle CPU.  Such condition is considered by
> asym_active_balance() in load balancing but not when looking for busiest
> group and load imbalance.  Add this consideration in find_busiest_group()
> and calculate_imbalance().
>
> Reviewed-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>

Could you have a look at what we did with misfit ?

we already have a SD_ASYM_CPUCAPACITY when  busiest->group_type ==
group_misfit_task. Misfit_task is between overloaded and ahs_spare to
handle such situation. Please detect this during the update of
statistic and tag the group correctly. We will not re-start to add
exception everywhere.

> ---
>  kernel/sched/fair.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 87317634fab2..bde962aa160a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9433,6 +9433,17 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>                                 sgs->group_capacity;
>  }
>
> +/* One group is SMT while the other group is not */
> +static inline bool asymmetric_groups(struct sched_group *sg1,
> +                                   struct sched_group *sg2)
> +{
> +       if (!sg1 || !sg2)
> +               return false;
> +
> +       return (sg1->flags & SD_SHARE_CPUCAPACITY) !=
> +               (sg2->flags & SD_SHARE_CPUCAPACITY);
> +}
> +
>  /**
>   * update_sd_pick_busiest - return 1 on busiest group
>   * @env: The load balancing environment.
> @@ -10079,6 +10090,31 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>         update_idle_cpu_scan(env, sum_util);
>  }
>
> +static inline bool asym_active_balance_busiest(struct lb_env *env, struct sd_lb_stats *sds)
> +{
> +       /*
> +        * Don't balance to a group without spare capacity.
> +        *
> +        * Skip non asymmetric sched group balancing. That check
> +        * is handled by code path handling imbalanced load between
> +        * similar groups.
> +        */
> +       if (env->idle == CPU_NOT_IDLE ||
> +           sds->local_stat.group_type != group_has_spare ||
> +           !asymmetric_groups(sds->local, sds->busiest))
> +               return false;
> +
> +       /*
> +        * For SMT source group, pull when there are two or more
> +        * tasks over-utilizing a core.
> +        */
> +       if (sds->busiest->flags & SD_SHARE_CPUCAPACITY &&
> +           sds->busiest_stat.sum_h_nr_running > 1)
> +               return true;
> +
> +       return false;
> +}
> +
>  /**
>   * calculate_imbalance - Calculate the amount of imbalance present within the
>   *                      groups of a given sched_domain during load balance.
> @@ -10164,6 +10200,12 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>                         return;
>                 }
>
> +               if (asym_active_balance_busiest(env, sds)) {
> +                       env->migration_type = migrate_task;
> +                       env->imbalance = 1;
> +                       return;
> +               }
> +
>                 if (busiest->group_weight == 1 || sds->prefer_sibling) {
>                         unsigned int nr_diff = busiest->sum_nr_running;
>                         /*
> @@ -10371,6 +10413,9 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>                          */
>                         goto out_balanced;
>
> +               if (asym_active_balance_busiest(env, &sds))
> +                       goto force_balance;
> +
>                 if (busiest->group_weight > 1 &&
>                     local->idle_cpus <= (busiest->idle_cpus + 1))
>                         /*
> --
> 2.32.0
>