linux-kernel - Re: [PATCH 3/3 v2] sched/fair: reduce cases for active balance

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Date:   Thu, 07 Jan 2021 11:26:47 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org
Cc:     Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 3/3 v2] sched/fair: reduce cases for active balance

On 07/01/21 11:33, Vincent Guittot wrote:
> Active balance is triggered for a number of voluntary cases like misfit
> or pinned tasks cases but also after that a number of load balance
> attempts failed to migrate a task. There is no need to use active load
> balance when the group is overloaded because an overloaded state means
> that there is at least one waiting task. Nevertheless, the waiting task
> is not selected and detached until the threshold becomes higher than its
> load. This threshold increases with the number of failed lb (see the
> condition if ((load >> env->sd->nr_balance_failed) > env->imbalance) in
> detach_tasks()) and the waiting task will end up to be selected after a
> number of attempts.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 45 +++++++++++++++++++++++----------------------
>  1 file changed, 23 insertions(+), 22 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a3515dea1afc..00ec5b901188 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9499,13 +9499,32 @@ asym_active_balance(struct lb_env *env)
>  }
>
>  static inline bool
> -voluntary_active_balance(struct lb_env *env)
> +imbalanced_active_balance(struct lb_env *env)
> +{
> +	struct sched_domain *sd = env->sd;
> +
> +	/*
> +	 * The imbalanced case includes the case of pinned tasks preventing a fair
> +	 * distribution of the load on the system but also the even distribution of the
> +	 * threads on a system with spare capacity
> +	 */

Do you mean s/imbalanced/migrate_task/? This part here will affect
group_imbalanced, group_asym_packing, and some others.

> +	if ((env->migration_type == migrate_task) &&
> +	    (sd->nr_balance_failed > sd->cache_nice_tries+2))
> +		return 1;
> +
> +	return 0;
> +}
> +