Re: [PATCH v5] cpuidle: Fix last_residency division

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 <redacted>
Bisected-by: Shilpasri G Bhat [off-list ref]
Signed-off-by: Shreyas B. Prabhu <redacted>
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This looks so much cleaner :)

Acked-by: Balbir Singh <bsingharora@gmail.com>

Balbir