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cpuidle: Ensure menu coefficients stay within domain

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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>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
tuukka.tikkanen@linaro.org 2014-02-24 08:29:33 +02:00 committed by Rafael J. Wysocki
parent 22695ab631
commit 7ac2643667
1 changed files with 3 additions and 0 deletions
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3
drivers/cpuidle/governors/menu.c
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@ -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];