linux-kernel - Re: [RFC][PATCH v2 3/5] cpuidle/teo: Simplify a little

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Message-ID: <ZMt9jooB4HV/5uXp@e126311.manchester.arm.com>
Date:   Thu, 3 Aug 2023 11:12:30 +0100
From:   Kajetan Puchalski <kajetan.puchalski@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     anna-maria@...utronix.de, rafael@...nel.org, tglx@...utronix.de,
        frederic@...nel.org, gautham.shenoy@....com,
        linux-kernel@...r.kernel.org, daniel.lezcano@...aro.org,
        linux-pm@...r.kernel.org, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, kajetan.puchalski@....com
Subject: Re: [RFC][PATCH v2 3/5] cpuidle/teo: Simplify a little

On Wed, Aug 02, 2023 at 03:24:34PM +0200, Peter Zijlstra wrote:
> Remove some of the early exit cases that rely on state_count, since we
> have the additional tick state. Declutters some of the next patches, can
> possibly be re-instated later if desired.

How does having that added tick state compensate for not checking the
amount of states that the governor can choose from in the first place?

[...]

> -	/* Check if there is any choice in the first place. */
> -	if (drv->state_count < 2) {
> -		idx = 0;
> -		goto end;
> -	}
> -	if (!dev->states_usage[0].disable) {
> -		idx = 0;
> -		if (drv->states[1].target_residency_ns > duration_ns)
> -			goto end;
> -	}
> -
> -	cpu_data->utilized = teo_cpu_is_utilized(dev->cpu, cpu_data);
> -	/*
> -	 * If the CPU is being utilized over the threshold and there are only 2
> -	 * states to choose from, the metrics need not be considered, so choose
> -	 * the shallowest non-polling state and exit.
> -	 */
> -	if (drv->state_count < 3 && cpu_data->utilized) {
> -		for (i = 0; i < drv->state_count; ++i) {
> -			if (!dev->states_usage[i].disable &&
> -			    !(drv->states[i].flags & CPUIDLE_FLAG_POLLING)) {
> -				idx = i;
> -				goto end;
> -			}
> -		}
> -	}

What exactly is the benefit of removing this part? On systems with 2
idle states this will just make the governor pointlessly execute the
metrics code when the result is already known regardless. Seems like
pure added overhead.

> -
>  	/*
>  	 * Find the deepest idle state whose target residency does not exceed
>  	 * the current sleep length and the deepest idle state not deeper than
> @@ -541,7 +512,7 @@ static int teo_select(struct cpuidle_dri
>  	 * If the CPU is being utilized over the threshold, choose a shallower
>  	 * non-polling state to improve latency
>  	 */
> -	if (cpu_data->utilized)
> +	if (teo_cpu_is_utilized(dev->cpu, cpu_data))
>  		idx = teo_find_shallower_state(drv, dev, idx, duration_ns, true);
>  
>  end:
> 
>