linux-kernel - Re: [PATCH v1] cpuidle: Add sanity check for exit latency and target residency

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Message-ID: <a8d3fe252f5ed9d513c026331eeabfe5971196b1.camel@linux.intel.com>
Date: Fri, 07 Nov 2025 20:47:20 +0200
From: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>, Linux PM
 <linux-pm@...r.kernel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Daniel Lezcano
	 <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v1] cpuidle: Add sanity check for exit latency and
 target residency

On Fri, 2025-11-07 at 19:19 +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Make __cpuidle_driver_init() sanitize CPU idle states so that the exit
> latency of a given state is not greater than its target residency which
> would break cpuidle assumptions.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/cpuidle/driver.c |   11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> --- a/drivers/cpuidle/driver.c
> +++ b/drivers/cpuidle/driver.c
> @@ -193,6 +193,16 @@ static void __cpuidle_driver_init(struct
>  			s->exit_latency_ns =  0;
>  		else
>  			s->exit_latency = div_u64(s->exit_latency_ns, NSEC_PER_USEC);
> +
> +		/*
> +		 * Ensure that the exit latency of a CPU idle state does not
> +		 * exceed its target residency which is assumed in cpuidle in
> +		 * multiple places.
> +		 */
> +		if (s->exit_latency_ns > s->target_residency_ns) {
> +			s->target_residency_ns = s->exit_latency_ns;
> +			s->target_residency = s->exit_latency;
> +		}

I suggest to error out instead of capping it. Because as soon as you
cap it, you may end up with the target residency of the next C-state. 


Just erroring out is very explicit, no surprises, and the table
provider will have to just fix the table.


Thanks, Artem.