Re: [PATCH v2 3/4] sched/fair: Wait before decaying max_newidle_lb_cost

[Peter Zijlstra]

On Fri, Oct 15, 2021 at 08:02:01PM +0200, Vincent Guittot wrote:
> On Fri, 15 Oct 2021 at 19:41, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Fri, Oct 15, 2021 at 02:46:53PM +0200, 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.
> >
> > I was more thinking something long these lines; ofcourse, no idea how
> > well it actually behaves.
> >
> > Index: linux-2.6/include/linux/sched/topology.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/sched/topology.h
> > +++ linux-2.6/include/linux/sched/topology.h
> > @@ -98,7 +98,6 @@ struct sched_domain {
> >
> >         /* idle_balance() stats */
> >         u64 max_newidle_lb_cost;
> > -       unsigned long next_decay_max_lb_cost;
> >
> >         u64 avg_scan_cost;              /* select_idle_sibling */
> >
> > Index: linux-2.6/kernel/sched/fair.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/sched/fair.c
> > +++ linux-2.6/kernel/sched/fair.c
> > @@ -10241,6 +10241,17 @@ void update_max_interval(void)
> >  }
> >
> >  /*
> > + * Asymmetric IIR filter, 1/4th down, 3/4th up.
> > + */
> > +static void update_newidle_cost(u64 *cost, u64 new)
> > +{
> > +       s64 diff = new - *cost;
> > +       if (diff > 0)
> > +               diff *= 3;
> > +       *cost += diff / 4;
> > +}
> 
> I tried to use something similar which was based on update_avg() but
> there were some performance regressions:
> some regressions were linked to not jumping to the new max directly. I
> assume some level were started whereas it would take too much time
> and some regressions happened  if the decay was too quick

Hmm, fair enough..

There's always something like:

       s64 diff = new - *cost;
       if (diff < 0)
               diff = 3*diff/256;
       *cost += diff;

Which jumps up instantly and decays rather slower. The advantage of
something like that, as I see it, is all those lines it deletes, but if
it doesn't actually work, it doesn't work.

A well. Thanks for trying.