[PATCH] cpuidle: Fix last_residency division

[Shreyas B Prabhu]

Please ignore this mail. I have resent this post using the correct
version number. Sorry for the noise.


On 06/30/2016 07:57 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 a better approximation for division by 1000.
> 
> Reported-by: Anton Blanchard <anton at samba.org>
> Bisected-by: Shilpasri G Bhat <shilpa.bhat at linux.vnet.ibm.com>
> Suggested-by David Laight <david.laight at aculab.com>
> Signed-off-by: Shreyas B. Prabhu <shreyas at linux.vnet.ibm.com>
> ---
> Changes in v4
> =============
>  - Increasing the threshold upto which approximation can be used.
>  - Removed explicit cast. Instead added a comment saying why cast
>    is safe.
> 
> Changes in v3
> =============
>  - Using approximation suggested by David
> 
> Changes in v2
> =============
>  - Fixing it in the cpuidle core code instead of driver code.
>