linux-kernel - Re: [PATCH 2/3 v2] sched/fair: don't set LBF_ALL

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Date:   Thu, 7 Jan 2021 17:00:53 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Tao Zhou <ouwen210@...mail.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>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3 v2] sched/fair: don't set LBF_ALL_PINNED unnecessarily

On Thu, 7 Jan 2021 at 16:08, Tao Zhou <ouwen210@...mail.com> wrote:
>
> Hi Vincent,
>
> On Thu, Jan 07, 2021 at 11:33:24AM +0100, 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.
> >
> > LBF_ALL_PINNED flag is now always set it by default. It is then cleared
> > when we find one task that can be pulled when calling detach_tasks() or
> > during active migration.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >  kernel/sched/fair.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 5428b8723e61..a3515dea1afc 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9626,6 +9626,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
> >       env.src_rq = busiest;
> >
> >       ld_moved = 0;
> > +     /* Clear this flag as soon as we find a pullable task */
> > +     env.flags |= LBF_ALL_PINNED;
> >       if (busiest->nr_running > 1) {
> >               /*
> >                * Attempt to move tasks. If find_busiest_group has found
> > @@ -9633,7 +9635,6 @@ 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:
> > @@ -9759,10 +9760,12 @@ 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;
>
> busiest->nr_running > 1, LBF_ALL_PINNED cleared but !ld_moved and get here.
> This is not consistent with the tip sched code because the original code
> from this path unconditionally set LBF_ALL_PINNED. But is this intentional
> to not increase balance interval and allow other tasks migrate not in the
> next balance interval.
>
> In v1, there was a condition here to allow that only one task running on rq
> can set LBF_ALL_PINNED. But in v2, when busiest->nr_running > 1, !ld_moved,
> LBF_ALL_PINNED is not cleared and can get here. Increase the balance interval.
> Not consist with v1. If I am not wrong, need a condition like:
>
>   if (busiest->nr_running != 1 /* && env.flags & LBF_ALL_PINNED */)
>       env.flags &= ~LBF_ALL_PINNED;

if (nr_running > 1) then LBF_ALL_PINNED can't be set when we reach the
active migration  (if (!ld_moved) { ...) because we go to
out_all_pinned if LBF_ALL_PINNED is set and we tried all cpus of the
sched_group

>
> I hope this is not a noise to this new thread.
>
> Thanks,
> Tao
>
> > once they become cache-cold as an example
>
> >                               goto out_one_pinned;
> >                       }
> >
> > +                     /* Record that we found atleast one task that could run on this_cpu */
> > +                     env.flags &= ~LBF_ALL_PINNED;
> > +
> >                       /*
> >                        * ->active_balance synchronizes accesses to
> >                        * ->active_balance_work.  Once set, it's cleared
> > --
> > 2.17.1
> >