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SubjectRe: [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@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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