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

[Hongyan Xia <hongyan.xia2@arm.com>]

On 28/08/2023 00:31, 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);

This looks a bit uncomfortable as the existing comment says that we assume:

	if (!load)
		runnable = running = 0;

I haven't looked at the math in detail, but if this is okay, maybe a 
comment saying why this is okay despite the existing comment says otherwise?

> +		___update_load_avg(&sa, 0);
> +		runtime++;
> +	}
> +
> +	return runtime;
> +} > [...]