[3/7] Cpuidle: Ensure menu coefficients stay within domain

Message ID 1393223377-5744-4-git-send-email-tuukka.tikkanen@linaro.org
State Accepted
Commit 7ac26436677024ab5e57e25c2c83d4c3fc232106
Headers show

Commit Message

Tuukka Tikkanen Feb. 24, 2014, 6:29 a.m.
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@linaro.org>
---
 drivers/cpuidle/governors/menu.c |    3 +++
 1 file changed, 3 insertions(+)

Comments

Nicolas Pitre Feb. 24, 2014, 4:57 p.m. | #1
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@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.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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Patch

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];