linux-kernel - Re: [PATCH 6/6] cpuidle: teo: Don't count non-existent intercepts

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Message-ID: <a71314e6-b248-44a4-8a82-0a8dfde9d44f@arm.com>
Date: Fri, 7 Jun 2024 12:17:32 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Christian Loehle <christian.loehle@....com>, linux-pm@...r.kernel.org,
 linux-kernel@...r.kernel.org, rafael@...nel.org
Cc: vincent.guittot@...aro.org, qyousef@...alina.io, peterz@...radead.org,
 daniel.lezcano@...aro.org, anna-maria@...utronix.de,
 kajetan.puchalski@....com, lukasz.luba@....com
Subject: Re: [PATCH 6/6] cpuidle: teo: Don't count non-existent intercepts

On 06/06/2024 11:00, Christian Loehle wrote:
> When bailing out early, teo will not query the sleep length anymore
> since commit 6da8f9ba5a87 ("cpuidle: teo:
> Skip tick_nohz_get_sleep_length() call in some cases") with an
> expected sleep_length_ns value of KTIME_MAX.
> This lead to state0 accumulating lots of 'intercepts' because
> the actually measured sleep length was < KTIME_MAX, so count KTIME_MAX
> as a hit (we have to count them as something otherwise we are stuck).
> 
> Fundamentally we can only do one of the two:
> 1. Skip sleep_length_ns query when we think intercept is likely
> 2. Have accurate data if sleep_length_ns is actually intercepted when
> we believe it is currently intercepted.
> 
> This patch chooses that latter as I've found the additional time it
> takes to query the sleep length to be negligible and the variants of
> option 1 (count all unknowns as misses or count all unknown as hits)
> had significant regressions (as misses had lots of too shallow idle
> state selections and as hits had terrible performance in
> intercept-heavy workloads).

So '2.' is the 'if (prev_intercept_idx != idx && !idx)' case ?

[...]

> @@ -514,6 +521,14 @@ static int teo_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>  			first_suitable_idx = i;
>  		}
>  	}
> +	if (prev_intercept_idx != idx && !idx) {

if (!idx && prev_intercept_idx) ?

> +		/*
> +		 * We have to query the sleep length here otherwise we don't
> +		 * know after wakeup if our guess was correct.
> +		 */
> +		duration_ns = tick_nohz_get_sleep_length(&delta_tick);
> +		cpu_data->sleep_length_ns = duration_ns;
> +	}
>  
>  	/*
>  	 * If there is a latency constraint, it may be necessary to select an