[PATCH v5] cpuidle: Fix last_residency division

[Daniel Lezcano]

On 07/01/2016 04:24 PM, Shreyas B. Prabhu wrote:
> Snooze is a poll idle state in powernv and pseries platforms. Snooze
> has a timeout so that if a cpu stays in snooze for more than target
> residency of the next available idle state, then it would exit thereby
> giving chance to the cpuidle governor to re-evaluate and
> promote the cpu to a deeper idle state. Therefore whenever snooze exits
> due to this timeout, its last_residency will be target_residency of next
> deeper state.
>
> commit e93e59ce5b85 ("cpuidle: Replace ktime_get() with local_clock()")
> changed the math around last_residency calculation. Specifically, while
> converting last_residency value from nanoseconds to microseconds it does
> right shift by 10. Due to this, in snooze timeout exit scenarios
> last_residency calculated is roughly 2.3% less than target_residency of
> next available state. This pattern is picked up get_typical_interval()
> in the menu governor and therefore expected_interval in menu_select() is
> frequently less than the target_residency of any state but snooze.
>
> Due to this we are entering snooze at a higher rate, thereby affecting
> the single thread performance.
>
> Fix this by using precise division via ktime_us_delta.
>
> Reported-by: Anton Blanchard <anton at samba.org>
> Bisected-by: Shilpasri G Bhat <shilpa.bhat at linux.vnet.ibm.com>
> Signed-off-by: Shreyas B. Prabhu <shreyas at linux.vnet.ibm.com>
> ---

Acked-by: Daniel Lezcano <daniel.lezcano at linaro.org>


-- 
  <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog