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

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Date:   Thu, 20 Dec 2018 15:33:51 +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 2/3] sched/fair: trigger asym_packing during idle load balance

On Thu, 20 Dec 2018 at 12:19, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> On 20/12/2018 07:55, 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 9b31247..487c73e 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8853,7 +8853,7 @@ 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
> >
>
> Regarding just extending the condition to include idle balance:
>
> Reviewed-by: Valentin Schneider <valentin.schneider@....com>
>
>
> As in the previous thread, I'll still argue that if you want to *reliably*
> exploit newidle balances to do asym packing active balancing, you should
> add some logic to raise rq->rd->overload when we notice some asym packing
> could be done, so that it can be leveraged by a higher-priority CPU doing
> a newidle balance.

The source cpu with the task is never overloaded.
We need to start the load balance to know that it's worth migrating the task.

>
> Otherwise the few newidle asym-packing active balances you'll get will be
> due to somewhat random luck because we happened to set that overload flag
> at some point.