Re: [RFC PATCH 03/16] sched/pelt: Add a new function to approximate runtime to reach given util

["Sultan Alsawaf (unemployed)" <sultan@kerneltoast.com>]

On Tue, Aug 20, 2024 at 05:34:59PM +0100, 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 <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 2ce83e880bd5..06cb881ba582 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;

Seems like this check can be removed since it's covered by the loop condition.

> +
> +	while (sa.util_avg < util) {
> +		accumulate_sum(delta, &sa, 1, 0, 1);
> +		___update_load_avg(&sa, 0);
> +		runtime++;
> +	}

I think this could be a lookup table (probably 1024 * u8), for constant-time
runtime approximation.

> +
> +	return runtime;
> +}
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 294c6769e330..47f158b2cdc2 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3065,6 +3065,7 @@ unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual,
>  				 unsigned long max);
>  
>  unsigned long approximate_util_avg(unsigned long util, u64 delta);
> +u64 approximate_runtime(unsigned long util);
>  
>  /*
>   * Verify the fitness of task @p to run on @cpu taking into account the
> -- 
> 2.34.1
> 

Cheers,
Sultan