Re: [PATCH v3] cpuidle: Fix last_residency division

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On 06/29/2016 09:06 AM, 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@samba.org>
> Bisected-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
> Suggested-by David Laight <david.laight@aculab.com>
> Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>

[Cc'ed Nicolas Pitre]

> ---
> Changes in v3
> =============
>   - Using approximation suggested by David
>
> Changes in v2
> =============
>   - Fixing it in the cpuidle core code instead of driver code.
>
>   drivers/cpuidle/cpuidle.c | 11 +++--------
>   drivers/cpuidle/cpuidle.h | 23 +++++++++++++++++++++++
>   2 files changed, 26 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index a4d0059..e9a7f74 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -174,7 +174,6 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
>   	struct cpuidle_state *target_state = &drv->states[index];
>   	bool broadcast = !!(target_state->flags & CPUIDLE_FLAG_TIMER_STOP);
>   	u64 time_start, time_end;
> -	s64 diff;
>
>   	/*
>   	 * Tell the time framework to switch to a broadcast timer because our
> @@ -218,14 +217,10 @@ 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, convert it to
> +	 * microsecond based time.
>   	 */
> -	diff = (time_end - time_start) >> 10;
> -	if (diff > INT_MAX)
> -		diff = INT_MAX;
> -
> -	dev->last_residency = (int) diff;
> +	dev->last_residency = convert_nsec_to_usec(time_end - time_start);
>
>   	if (entered_state >= 0) {
>   		/* Update cpuidle counters */
> diff --git a/drivers/cpuidle/cpuidle.h b/drivers/cpuidle/cpuidle.h
> index f87f399..c8ea5ad 100644
> --- a/drivers/cpuidle/cpuidle.h
> +++ b/drivers/cpuidle/cpuidle.h
> @@ -68,4 +68,27 @@ static inline void cpuidle_coupled_unregister_device(struct cpuidle_device *dev)
>   }
>   #endif
>
> +/*
> + * Used for calculating last_residency in usec. Optimized for case
> + * where last_residency in nsecs is < INT_MAX/2 by using faster
> + * approximation. Approximated value has less than 1% error.
> + */
> +static inline int convert_nsec_to_usec(u64 nsec)
> +{
> +	if (likely(nsec < INT_MAX / 2)) {

UINT_MAX ?

> +		int usec = (int)nsec;
> +
> +		usec += usec >> 5;
> +		usec = usec >> 10;
> +		return usec;
> +	} else {
> +		u64 usec = div_u64(nsec, 1000);
> +
> +		if (usec > INT_MAX)
> +			usec = INT_MAX;
> +		return (int)usec;
> +	}
> +}



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