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

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Date:   Tue, 18 Dec 2018 09:17:44 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH v2 2/3] sched/fair: trigger asym_packing during idle load balance

On Mon, 17 Dec 2018 at 17:59, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> 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:

It's not needed in this case because the dst cpu is already the target
cpu and the migration will happen during this idle load balance.
Setting root_domain->overload is useful only if you want another cpu
to pull the task during another coming load_balance (newly or normal
idle ones) which is not the case here.

>
> -----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).