Re: [RFC PATCH 1/7] sched/pelt: Add a new function to approximate the future util_avg value

Qais Yousef
2023-09-06T21:19:44Z

On 09/06/23 14:56, Dietmar Eggemann wrote:
> On 28/08/2023 01:31, Qais Yousef wrote:
> > Given a util_avg value, the new function will return the future one
> > given a runtime delta.
> >
> > This will be useful in later patches to help replace some magic margins
> > with more deterministic behavior.
> >
> > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > ---
> > kernel/sched/pelt.c | 22 +++++++++++++++++++++-
> > kernel/sched/sched.h | 3 +++
> > 2 files changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> > index 0f310768260c..50322005a0ae 100644
> > --- a/kernel/sched/pelt.c
> > +++ b/kernel/sched/pelt.c
> > @@ -466,4 +466,24 @@ int update_irq_load_avg(struct rq *rq, u64 running)
> >
> > return ret;
> > }
> > -#endif
> > +#endif /* CONFIG_HAVE_SCHED_AVG_IRQ */
> > +
> > +/*
> > + * Approximate the new util_avg value assuming an entity has continued to run
> > + * for @delta us.
> > + */
> > +unsigned long approximate_util_avg(unsigned long util, u64 delta)
> > +{
> > + struct sched_avg sa = {
> > + .util_sum = util * PELT_MIN_DIVIDER,
> > + .util_avg = util,
> > + };
> > +
> > + if (unlikely(!delta))
> > + return util;
> > +
> > + accumulate_sum(delta, &sa, 0, 0, 1);
>
> IMHO, you miss the handling of `periods != 0`. load = 0 eclipses this
> code in accumulate_sum().

Yes. For some reason I got blank registered when I saw if this codepath can
impact util_avg..

>
> > + ___update_load_avg(&sa, 0);
> > +
> > + return sa.util_avg;
> > +}
>
> We already discussed something similar like this in Nov 22, the so
> called UTIL_EST_FASTER thing.
>
> https://lkml.kernel.org/r/Y2kLA8x40IiBEPYg@hirez.programming.kicks-ass.net
>
> +/*
> + * Compute a pelt util_avg assuming no history and @delta runtime.
> + */
> +unsigned long faster_est_approx(u64 delta)
> +{
> + unsigned long contrib = (unsigned long)delta; /* p == 0 -> delta < 1024 */
> + u64 periods = delta / 1024;
> +
> + if (periods) {
> + delta %= 1024;
> + contrib = __accumulate_pelt_segments(periods, 1024, delta);
> + }
> +
> + return (contrib << SCHED_CAPACITY_SHIFT) / PELT_MIN_DIVIDER;
> +}
> +

I could look at using this version instead. This misses the decay part though?

Thanks!

--
Qais Yousef