linux-kernel - Re: [PATCH v1 1/4] cpuidle: governors: teo: Drop incorrect target residency check

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Message-ID: <1dc4934f-a3ce-4ead-a43c-0a80987364b6@arm.com>
Date: Thu, 13 Nov 2025 11:32:42 +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 v1 1/4] cpuidle: governors: teo: Drop incorrect target
 residency check

On 11/12/25 16:22, 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 it does not make sense to
> select an idle state with target residency above the sleep length.
> 
> Accordingly, drop the teo_state_ok() check done in that case 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>
> ---
>  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
> 
> 
> 

AFAICT this check was to not be stuck in a shallow state when tick is already disabled.
There might be a timer armed in t+500us but that might still get cancelled, which
is why we didn't think a below TICK_NSEC 'shallow' state is acceptable?