Re: [PATCH 3/5] sched/fair: Wait before decaying max_newidle_lb_cost

[Vincent Guittot]

On Fri, 29 Oct 2021 at 12:01, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
>
> On 19/10/2021 14:35, Vincent Guittot wrote:
> > Decay max_newidle_lb_cost only when it has not been updated for a while
> > and ensure to not decay a recently changed value.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > ---
> >  include/linux/sched/topology.h |  2 +-
> >  kernel/sched/fair.c            | 36 +++++++++++++++++++++++++---------
> >  kernel/sched/topology.c        |  2 +-
> >  3 files changed, 29 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> > index 2f9166f6dec8..c07bfa2d80f2 100644
> > --- a/include/linux/sched/topology.h
> > +++ b/include/linux/sched/topology.h
> > @@ -105,7 +105,7 @@ struct sched_domain {
> >
> >       /* idle_balance() stats */
> >       u64 max_newidle_lb_cost;
> > -     unsigned long next_decay_max_lb_cost;
> > +     unsigned long last_decay_max_lb_cost;
> >
> >       u64 avg_scan_cost;              /* select_idle_sibling */
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index c4c36865321b..e50fd751e1df 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -10239,6 +10239,30 @@ void update_max_interval(void)
> >       max_load_balance_interval = HZ*num_online_cpus()/10;
> >  }
> >
> > +static inline bool update_newidle_cost(struct sched_domain *sd, u64 cost)
> > +{
> > +     if (cost > sd->max_newidle_lb_cost) {
> > +             /*
> > +              * Track max cost of a domain to make sure to not delay the
> > +              * next wakeup on the CPU.
> > +              */
> > +             sd->max_newidle_lb_cost = cost;
> > +             sd->last_decay_max_lb_cost = jiffies;
>
> That's the actual change of the patch: sd->last_decay_max_lb_cost being
> moved forward also when newidle cost is updated from newidle_balance() ?
>
> > +     } else if (time_after(jiffies, sd->last_decay_max_lb_cost + HZ)) {
> > +             /*
> > +              * Decay the newidle max times by ~1% per second to ensure that
> > +              * it is not outdated and the current max cost is actually
> > +              * shorter.
>
> I assume that `max cost` refers here to a local variable of the only
> caller of update_newidle_cost(..., 0) - rebalance_domains()?
>
> "the current max cost" has to be shorter so that
> rq->max_idle_balance_cost also decays in this case. Is this what this
> comment tries to say here?

I refer to the time tracked in sd->max_newidle_lb_cost
here i set current_cost to zero to trigger a possible decay of
sd->max_newidle_lb_cost

>
> [...]