linux-kernel - Re: [PATCH 3/7] Cpuidle: Ensure menu coefficients stay within domain

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Date:	Mon, 24 Feb 2014 11:57:14 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Tuukka Tikkanen <tuukka.tikkanen@...aro.org>
cc:	linux-pm@...r.kernel.org, rjw@...ysocki.net,
	daniel.lezcano@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] Cpuidle: Ensure menu coefficients stay within
 domain

On Mon, 24 Feb 2014, Tuukka Tikkanen wrote:

> The menu governor uses coefficients as one method of actual idle
> period length estimation. The coefficients are, as detailed below,
> multipliers giving expected idle period length from time until next
> timer expiry. The multipliers are supposed to have domain of (0..1].
> 
> The coefficients are fractions where only the numerators are stored
> and denominators are a shared constant RESOLUTION*DECAY. Since the
> value of the coefficient should always be greater than 0 and less
> than or equal to 1, the numerator must have a value greater than
> 0 and less than or equal to RESOLUTION*DECAY.
> 
> If the coefficients are updated with measured idle durations exceeding
> timer length, the multiplier may reach values exceeding unity (i.e.
> the stored numerator exceeds RESOLUTION*DECAY). This patch ensures that
> the multipliers are updated with durations capped to timer length.
> 
> Signed-off-by: Tuukka Tikkanen <tuukka.tikkanen@...aro.org>
Acked-by: Nicolas Pitre <nico@...aro.org>


> ---
>  drivers/cpuidle/governors/menu.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
> index 115386a..f0995dd 100644
> --- a/drivers/cpuidle/governors/menu.c
> +++ b/drivers/cpuidle/governors/menu.c
> @@ -410,6 +410,9 @@ static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
>  	if (measured_us > target->exit_latency)
>  		measured_us -= target->exit_latency;
>  
> +	/* Make sure our coefficients do not exceed unity */
> +	if (measured_us > data->next_timer_us)
> +		measured_us = data->next_timer_us;
>  
>  	/* Update our correction ratio */
>  	new_factor = data->correction_factor[data->bucket];
> -- 
> 1.7.9.5
> 
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