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Message-ID: <bad29fb0-c734-853b-492b-ce2d01a293c5@arm.com>
Date:   Wed, 6 Sep 2023 14:56:57 +0200
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Subject: Re: [RFC PATCH 2/7] sched/pelt: Add a new function to approximate
 runtime to reach given util
Content-Language: en-US
To:     Qais Yousef <qyousef@layalina.io>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        Lukasz Luba <lukasz.luba@arm.com>
References: <20230827233203.1315953-1-qyousef@layalina.io>
 <20230827233203.1315953-3-qyousef@layalina.io>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
In-Reply-To: <20230827233203.1315953-3-qyousef@layalina.io>
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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

[...]