Re: [RFC PATCH 2/7] sched/pelt: Add a new function to approximate runtime to reach given util

Dietmar Eggemann
2023-09-06T12:57:14Z

On 28/08/2023 01:31, Qais Yousef wrote:
> It is basically the ramp-up time from 0 to a given value. Will be used
> later to implement new tunable to control response time for schedutil.
>
> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> ---
> kernel/sched/pelt.c | 21 +++++++++++++++++++++
> kernel/sched/sched.h | 1 +
> 2 files changed, 22 insertions(+)
>
> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> index 50322005a0ae..f673b9ab92dc 100644
> --- a/kernel/sched/pelt.c
> +++ b/kernel/sched/pelt.c
> @@ -487,3 +487,24 @@ unsigned long approximate_util_avg(unsigned long util, u64 delta)
>
> return sa.util_avg;
> }
> +
> +/*
> + * Approximate the required amount of runtime in ms required to reach @util.
> + */
> +u64 approximate_runtime(unsigned long util)
> +{
> + struct sched_avg sa = {};
> + u64 delta = 1024; // period = 1024 = ~1ms
> + u64 runtime = 0;
> +
> + if (unlikely(!util))
> + return runtime;
> +
> + while (sa.util_avg < util) {
> + accumulate_sum(delta, &sa, 0, 0, 1);
> + ___update_load_avg(&sa, 0);
> + runtime++;
> + }
> +
> + return runtime;
> +}

S_n = S_inv * (1 - 0.5^(t/hl))

t = hl * ln(1 - Sn/S_inv)/ln(0.5)

(1) for a little CPU (capacity_orig = 446)

t = 32ms * ln(1 - 446/1024)/ln(0.5)

t = 26ms

(2) for a big CPU (capacity = 1023 (*instead of 1024 since ln(0) not
defined

t = 32ms * ln(1 - 1023/1024)/ln(0.5)

t = 320ms …