RE: [PATCH v2] cpuidle: Fix last_residency division

[David Laight <David.Laight@ACULAB.COM>]

From: Shreyas B. Prabhu
> 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 <anton@samba.org>
> Bisected-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
> Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
> ---
> 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;
>  	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.

	David

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