[PATCH v5] cpuidle: Fix last_residency division

[Rafael J. Wysocki]

On Mon, Jul 4, 2016 at 8:10 AM, Balbir Singh <bsingharora at gmail.com> wrote:
>
>
> On 02/07/16 00:24, 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>
>> ---
>
> This looks so much cleaner :)
>
> Acked-by: Balbir Singh <bsingharora at gmail.com>

Patch applied, thanks!