linux-kernel - Re: [PATCH v2 2/3] sched/fair: trigger asym

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Date:   Mon, 17 Dec 2018 16:59:39 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
        mingo@...nel.org, linux-kernel@...r.kernel.org
Cc:     Morten.Rasmussen@....com
Subject: Re: [PATCH v2 2/3] sched/fair: trigger asym_packing during idle load
 balance

Hi Vincent,

On 14/12/2018 16:01, Vincent Guittot wrote:
> newly idle load balance is not always triggered when a cpu becomes idle.
> This prevent the scheduler to get a chance to migrate task for asym packing.
> Enable active migration because of asym packing during idle load balance too.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c215f7a..9591e7a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8861,7 +8861,7 @@ static int need_active_balance(struct lb_env *env)
>  {
>  	struct sched_domain *sd = env->sd;
>  
> -	if (env->idle == CPU_NEWLY_IDLE) {
> +	if (env->idle != CPU_NOT_IDLE) {
>  
>  		/*
>  		 * ASYM_PACKING needs to force migrate tasks from busy but
> 

That change looks fine. However, you're mentioning newidle load_balance()
not being triggered - you'd want to set root_domain->overload for any
newidle pull to happen, probably with something like this:

-----8<-----
@@ -8398,6 +8408,9 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
                sg = sg->next;
        } while (sg != env->sd->groups);
 
+       if (check_asym_packing(env, sds))
+               sg_status |= SG_OVERLOAD;
+
 #ifdef CONFIG_NO_HZ_COMMON
        if ((env->flags & LBF_NOHZ_AGAIN) &&
            cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) {
----->8-----

It's similar to what is done for misfit, although that's yet another
'twisted' use of that flag which we might want to rename (I suggested
something like 'need_idle_balance' a while back but it wasn't really
popular).