linux-kernel - Re: [PATCH v2 1/4] cpuidle: governors: teo: Drop misguided target residency check

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Message-ID: <2d8f3f25-1b96-4e66-83b8-78a3b501f1e4@arm.com>
Date: Fri, 14 Nov 2025 09:16:45 +0000
From: Christian Loehle <christian.loehle@....com>
To: "Rafael J. Wysocki" <rafael@...nel.org>,
 Linux PM <linux-pm@...r.kernel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Reka Norman <rekanorman@...omium.org>
Subject: Re: [PATCH v2 1/4] cpuidle: governors: teo: Drop misguided target
 residency check

On 11/13/25 13:24, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> When the target residency of the current candidate idle state is
> greater than the expected time till the closest timer (the sleep
> length), it does not matter whether or not the tick has already been
> stopped or if it is going to be stopped.  The closest timer will
> trigger anyway at its due time, so if an idle state with target
> residency above the sleep length is selected, energy will be wasted
> and there may be excess latency.
> 
> Of course, if the closest timer were canceled before it could trigger,
> a deeper idle state would be more suitable, but this is not expected
> to happen (generally speaking, hrtimers are not expected to be
> canceled as a rule).
> 
> Accordingly, the teo_state_ok() check done in that case causes energy to
> be wasted more often than it allows any energy to be saved (if it allows
> any energy to be saved at all), so drop it and let the governor use the
> teo_find_shallower_state() return value as the new candidate idle state
> index.
> 
> Fixes: 21d28cd2fa5f ("cpuidle: teo: Do not call tick_nohz_get_sleep_length() upfront")
> Cc: All applicable <stable@...r.kernel.org>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> 
> v1 -> v2: Subject and changelog modifications
> 
> ---
>  drivers/cpuidle/governors/teo.c |    7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> --- a/drivers/cpuidle/governors/teo.c
> +++ b/drivers/cpuidle/governors/teo.c
> @@ -458,11 +458,8 @@ static int teo_select(struct cpuidle_dri
>  	 * If the closest expected timer is before the target residency of the
>  	 * candidate state, a shallower one needs to be found.
>  	 */
> -	if (drv->states[idx].target_residency_ns > duration_ns) {
> -		i = teo_find_shallower_state(drv, dev, idx, duration_ns, false);
> -		if (teo_state_ok(i, drv))
> -			idx = i;
> -	}
> +	if (drv->states[idx].target_residency_ns > duration_ns)
> +		idx = teo_find_shallower_state(drv, dev, idx, duration_ns, false);
>  
>  	/*
>  	 * If the selected state's target residency is below the tick length
> 

Reviewed-by: Christian Loehle <christian.loehle@....com>