linux-kernel - Re: [PATCH v2 8/8] sched/fair: use utilization to select misfit task

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Date:   Fri, 2 Aug 2019 10:29:06 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Phil Auld <pauld@...hat.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH v2 8/8] sched/fair: use utilization to select misfit task

On Thu, 1 Aug 2019 at 18:27, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> On 01/08/2019 15:40, Vincent Guittot wrote:
> > @@ -8261,7 +8261,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> >                * If we have more than one misfit sg go with the
> >                * biggest misfit.
> >                */
> > -             if (sgs->group_misfit_task_load < busiest->group_misfit_task_load)
> > +             if (sgs->group_misfit_task_util < busiest->group_misfit_task_util)
> >                       return false;
>
> I previously said this change would render the maximization useless, but I
> had forgotten one thing: with PELT time scaling, task utilization can go
> above its CPU's capacity.
>
> So if you have two LITTLE CPUs running a busy loop (misfit task) each, the
> one that's been running the longest would have the highest utilization
> (assuming they haven't reached 1024 yet). In other words "utilizations
> above the capacity_margin can't be compared" doesn't really stand.
>
> Still, maximizing load would lead us there. Furthermore, if we have to pick
> between two rqs with misfit tasks, I still believe we should pick the one
> with the highest load, not the highest utilization.
>
> We could keep load and fix the issue of detaching the wrong task with
> something like:
>
> -----8<-----
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 53e64a7b2ae0..bfc2b624ee98 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7489,12 +7489,8 @@ static int detach_tasks(struct lb_env *env)
>                 case migrate_misfit:
>                         load = task_h_load(p);
>
> -                       /*
> -                        * utilization of misfit task might decrease a bit
> -                        * since it has been recorded. Be conservative in the
> -                        * condition.
> -                        */
> -                       if (load < env->imbalance)
> +                       /* This is not a misfit task */
> +                       if (task_fits_capacity(p, capacity_of(env->src_cpu)))
>                                 goto next;

This could be a solution for make sure to pull only misfit task and
keep using load

>
>                         env->imbalance = 0;
> ----->8-----
>
> However what would be *even* better IMO would be:
>
> -----8<-----
> @@ -8853,6 +8853,7 @@ voluntary_active_balance(struct lb_env *env)
>                         return 1;
>         }
>
> +       /* XXX: make sure current is still a misfit task? */
>         if (env->balance_type == migrate_misfit)
>                 return 1;
>
> @@ -8966,6 +8967,20 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>         env.src_rq = busiest;
>
>         ld_moved = 0;
> +
> +       /*
> +        * Misfit tasks are only misfit if they are currently running, see
> +        * update_misfit_status().
> +        *
> +        * - If they're not running, we'll get an opportunity at wakeup to
> +        *   migrate them to a bigger CPU.
> +        * - If they're running, we need to active balance them to a bigger CPU.
> +        *
> +        * Do the smart thing and go straight for active balance.
> +        */
> +       if (env->balance_type == migrate_misfit)
> +               goto active_balance;
> +

This looks ugly and add a new bypass which this patchset tries to remove
This doesn't work if your misfit task has been preempted by another
one during the load balance and waiting for the runqueue


>         if (busiest->nr_running > 1) {
>                 /*
>                  * Attempt to move tasks. If find_busiest_group has found
> @@ -9074,7 +9089,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>                         goto out_all_pinned;
>                 }
>         }
> -
> +active_balance:
>         if (!ld_moved) {
>                 schedstat_inc(sd->lb_failed[idle]);
>                 /*
> ----->8-----