linux-kernel - Re: [PATCH 2/3] sched/fair: don't set LBF_ALL

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Date:   Wed, 6 Jan 2021 17:04:20 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] sched/fair: don't set LBF_ALL_PINNED unnecessarily

On Wed, 6 Jan 2021 at 16:13, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> On 06/01/21 14:34, Vincent Guittot wrote:
> > Setting LBF_ALL_PINNED during active load balance is only valid when there
> > is only 1 running task on the rq otherwise this ends up increasing the
> > balance interval whereas other tasks could migrate after the next interval
> > once they become cache-cold as an example.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >  kernel/sched/fair.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 5428b8723e61..69a455113b10 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9759,7 +9759,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
> >                       if (!cpumask_test_cpu(this_cpu, busiest->curr->cpus_ptr)) {
> >                               raw_spin_unlock_irqrestore(&busiest->lock,
> >                                                           flags);
> > -                             env.flags |= LBF_ALL_PINNED;
> > +                             if (busiest->nr_running == 1)
> > +                                     env.flags |= LBF_ALL_PINNED;
>
> So LBF_ALL_PINNED *can* be set if busiest->nr_running > 1, because
> before we get there we have:
>
>   if (nr_running > 1) {
>       env.flags |= LBF_ALL_PINNED;
>       detach_tasks(&env); // Removes LBF_ALL_PINNED if > 0 tasks can be pulled
>       ...
>   }
>
> What about following the logic used by detach_tasks() and only clear the
> flag? Say something like the below. if nr_running > 1, then we'll have
> gone through detach_tasks() and will have cleared the flag (if
> possible).
> ---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 04a3ce20da67..211c86ba3f5b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9623,6 +9623,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>         env.src_rq = busiest;
>
>         ld_moved = 0;
> +       /* Clear this as soon as we find a single pullable task */
> +       env.flags |= LBF_ALL_PINNED;
>         if (busiest->nr_running > 1) {
>                 /*
>                  * Attempt to move tasks. If find_busiest_group has found
> @@ -9630,7 +9632,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>                  * still unbalanced. ld_moved simply stays zero, so it is
>                  * correctly treated as an imbalance.
>                  */
> -               env.flags |= LBF_ALL_PINNED;
>                 env.loop_max  = min(sysctl_sched_nr_migrate, busiest->nr_running);
>
>  more_balance:
> @@ -9756,10 +9757,11 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>                         if (!cpumask_test_cpu(this_cpu, busiest->curr->cpus_ptr)) {
>                                 raw_spin_unlock_irqrestore(&busiest->lock,
>                                                             flags);
> -                               env.flags |= LBF_ALL_PINNED;
>                                 goto out_one_pinned;
>                         }
>
> +                       env.flags &= ~LBF_ALL_PINNED;

Yes, looks easier to read.
will do the change in the next version

> +
>                         /*
>                          * ->active_balance synchronizes accesses to
>                          * ->active_balance_work.  Once set, it's cleared
> ---