linux-kernel - Re: [PATCH v1 2/4] cpufreq: schedutil: Adjust utilization instead of frequency

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Date:   Tue, 8 Dec 2020 14:21:46 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Doug Smythies <dsmythies@...us.net>,
        Giovanni Gherdovich <ggherdovich@...e.com>
Subject: Re: [PATCH v1 2/4] cpufreq: schedutil: Adjust utilization instead of
 frequency

On 07-12-20, 17:29, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> When avoiding reduction of the frequency after the target CPU has
> been busy since the previous frequency update, adjust the utilization
> instead of adjusting the frequency, because doing so is more prudent
> (it is done to counter a possible utilization deficit after all) and
> it will allow some code to be shared after a subsequent change.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  kernel/sched/cpufreq_schedutil.c |   11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> Index: linux-pm/kernel/sched/cpufreq_schedutil.c
> ===================================================================
> --- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
> +++ linux-pm/kernel/sched/cpufreq_schedutil.c
> @@ -437,7 +437,7 @@ static void sugov_update_single(struct u
>  {
>  	struct sugov_cpu *sg_cpu = container_of(hook, struct sugov_cpu, update_util);
>  	struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> -	unsigned int cached_freq = sg_policy->cached_raw_freq;
> +	unsigned long prev_util = sg_cpu->util;
>  	unsigned int next_f;
>  
>  	sugov_iowait_boost(sg_cpu, time, flags);
> @@ -451,17 +451,14 @@ static void sugov_update_single(struct u
>  	sugov_get_util(sg_cpu);
>  	sugov_iowait_apply(sg_cpu, time);
>  
> -	next_f = get_next_freq(sg_policy, sg_cpu->util, sg_cpu->max);
>  	/*
>  	 * Do not reduce the frequency if the CPU has not been idle
>  	 * recently, as the reduction is likely to be premature then.
>  	 */
> -	if (sugov_cpu_is_busy(sg_cpu) && next_f < sg_policy->next_freq) {
> -		next_f = sg_policy->next_freq;
> +	if (sugov_cpu_is_busy(sg_cpu) && sg_cpu->util < prev_util)
> +		sg_cpu->util = prev_util;
>  
> -		/* Restore cached freq as next_freq has changed */
> -		sg_policy->cached_raw_freq = cached_freq;
> -	}
> +	next_f = get_next_freq(sg_policy, sg_cpu->util, sg_cpu->max);

I don't think we can replace freq comparison by util, or at least it will give
us a different final frequency and the behavior is changed.

Lets take an example, lets say current freq is 1 GHz and max is 1024.

Round 1: Lets say util is 1000

next_f = 1GHz * 1.25 * 1000/1024 = 1.2 GHz

Round 2: Lets say util has come down to 900 here,

before the patch:

next_f = 1.2 GHz * 1.25 * 900/1024 = 1.31 GHz

after the patch:

next_f = 1.2 GHz * 1.25 * 1000/1024 = 1.45 GHz

Or did I make a mistake here ?

-- 
viresh