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

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

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().

> +	___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;
+}
+

[...]