linux-kernel - Re: [PATCH v3 04/10] sched/fair: rework load

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Date:   Wed, 16 Oct 2019 12:51:34 +0530
From:   Parth Shah <parth@...ux.ibm.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org
Cc:     pauld@...hat.com, valentin.schneider@....com,
        srikar@...ux.vnet.ibm.com, quentin.perret@....com,
        dietmar.eggemann@....com, Morten.Rasmussen@....com,
        hdanton@...a.com
Subject: Re: [PATCH v3 04/10] sched/fair: rework load_balance



On 9/19/19 1:03 PM, Vincent Guittot wrote:

[...]

> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 585 ++++++++++++++++++++++++++++++++++------------------
>  1 file changed, 380 insertions(+), 205 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 017aad0..d33379c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7078,11 +7078,26 @@ static unsigned long __read_mostly max_load_balance_interval = HZ/10;
> 
>  enum fbq_type { regular, remote, all };
> 
> +/*
> + * group_type describes the group of CPUs at the moment of the load balance.
> + * The enum is ordered by pulling priority, with the group with lowest priority
> + * first so the groupe_type can be simply compared when selecting the busiest
> + * group. see update_sd_pick_busiest().
> + */
>  enum group_type {
> -	group_other = 0,
> +	group_has_spare = 0,
> +	group_fully_busy,
>  	group_misfit_task,
> +	group_asym_packing,
>  	group_imbalanced,
> -	group_overloaded,
> +	group_overloaded
> +};
> +
> +enum migration_type {
> +	migrate_load = 0,
> +	migrate_util,
> +	migrate_task,
> +	migrate_misfit
>  };
> 
>  #define LBF_ALL_PINNED	0x01
> @@ -7115,7 +7130,7 @@ struct lb_env {
>  	unsigned int		loop_max;
> 
>  	enum fbq_type		fbq_type;
> -	enum group_type		src_grp_type;
> +	enum migration_type	balance_type;
>  	struct list_head	tasks;
>  };
> 
> @@ -7347,7 +7362,7 @@ static int detach_tasks(struct lb_env *env)
>  {
>  	struct list_head *tasks = &env->src_rq->cfs_tasks;
>  	struct task_struct *p;
> -	unsigned long load;
> +	unsigned long util, load;
>  	int detached = 0;
> 
>  	lockdep_assert_held(&env->src_rq->lock);
> @@ -7380,19 +7395,53 @@ static int detach_tasks(struct lb_env *env)
>  		if (!can_migrate_task(p, env))
>  			goto next;
> 
> -		load = task_h_load(p);
> +		switch (env->balance_type) {
> +		case migrate_load:
> +			load = task_h_load(p);
> 
> -		if (sched_feat(LB_MIN) && load < 16 && !env->sd->nr_balance_failed)
> -			goto next;
> +			if (sched_feat(LB_MIN) &&
> +			    load < 16 && !env->sd->nr_balance_failed)
> +				goto next;
> 
> -		if ((load / 2) > env->imbalance)
> -			goto next;
> +			if ((load / 2) > env->imbalance)
> +				goto next;
> +
> +			env->imbalance -= load;
> +			break;
> +
> +		case migrate_util:
> +			util = task_util_est(p);
> +
> +			if (util > env->imbalance)

Can you please explain what would happen for
`if (util/2 > env->imbalance)` ?
just like when migrating load, even util shouldn't be migrated if
env->imbalance is just near the utilization of the task being moved, isn't it?

> +				goto next;
> +
> +			env->imbalance -= util;
> +			break;
> +[ ... ]

Thanks,
Parth