Re: [PATCH v2] cpuidle: Fix last_residency division

On 06/24/2016 11:00 AM, David Laight wrote:

From: Shreyas B. Prabhu

quoted

Sent: 24 June 2016 09:24

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 replacing right shift by 10 with /1000 while calculating
last_residency.

Reported-by: Anton Blanchard <redacted>
Bisected-by: Shilpasri G Bhat [off-list ref]
Signed-off-by: Shreyas B. Prabhu <redacted>
---
Changes in v2
=============
  - Fixing it in the cpuidle core code instead of driver code.

  drivers/cpuidle/cpuidle.c | 6 +++---
  1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index a4d0059..30d67a8 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c

@@ -218,10 +218,10 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
  		local_irq_enable();

  	/*
-	 * local_clock() returns the time in nanosecond, let's shift
-	 * by 10 (divide by 1024) to have microsecond based time.
+	 * local_clock() returns the time in nanosecond, let's
+	 * divide by 1000 to have microsecond based time.
  	 */
-	diff = (time_end - time_start) >> 10;
+	diff = (time_end - time_start) / 1000;

do_div ?

quoted

  	if (diff > INT_MAX)
  		diff = INT_MAX;

The intent of the >> 10 was probably to avoid an expensive 64bit divide.
So maybe something like:
	diff = time_end - time_start;
	if (diff >= INT_MAX/2)
		diff_32 = INT_MAX/2/1000;
	else
		diff_32 = diff;
		diff_32 += diff_32 >> 6;
		diff_32 >>= 10;
	}

Adding an extra 1/32 makes the division by be something slightly below 1000.

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