linux-kernel - Re: [RFC PATCH 5/7] sched/schedutil: Add a new tunable to dictate response time

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Message-ID: <20230907114420.GC10955@noisy.programming.kicks-ass.net>
Date:   Thu, 7 Sep 2023 13:44:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Lukasz Luba <lukasz.luba@....com>
Subject: Re: [RFC PATCH 5/7] sched/schedutil: Add a new tunable to dictate
 response time

On Mon, Aug 28, 2023 at 12:32:01AM +0100, Qais Yousef wrote:
> +static inline unsigned long
> +sugov_apply_response_time(struct sugov_policy *sg_policy, unsigned long util)
> +{
> +	unsigned long mult;
> +
> +	if (sg_policy->freq_response_time_ms == sg_policy->tunables->response_time_ms)
> +		return util;
> +
> +	mult = sg_policy->freq_response_time_ms * SCHED_CAPACITY_SCALE;
> +	mult /=	sg_policy->tunables->response_time_ms;
> +	mult *= util;
> +
> +	return mult >> SCHED_CAPACITY_SHIFT;
> +}
> +
>  static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>  {
>  	s64 delta_ns;
> @@ -143,6 +184,7 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
>  	unsigned int freq = arch_scale_freq_invariant() ?
>  				policy->cpuinfo.max_freq : policy->cur;
>  
> +	util = sugov_apply_response_time(sg_policy, util);
>  	freq = map_util_freq(util, freq, max);
>  
>  	if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)

Urgh, so instead of caching the multiplier you keep computing what is
essentially a constant over and over and over and over again :/

That is, compute the whole 'freq_response_time_ms * SCHED_CAPACITY_SCALE
/ response_time_ms' thing *once*, when that file is written to, and then
reduce the whole thing to:

	return (freq_response_mult * util) >> SCHED_CAPACITY_SHIFT;

No need for that special case, no need for divisions, just go.