Subject: Re: [PATCH v4] cpuidle: Fix last_residency division

From: Daniel Lezcano <[email-masked]>
Date: Thu, 30 Jun 2016 16:57:34 +0200

On 06/30/2016 04:34 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 <[email-masked]>
> Bisected-by: Shilpasri G Bhat <[email-masked]>
> Suggested-by David Laight <[email-masked]>
> Signed-off-by: Shreyas B. Prabhu <[email-masked]>
> ---
> 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.
>
>   drivers/cpuidle/cpuidle.c | 11 +++--------
>   drivers/cpuidle/cpuidle.h | 38 ++++++++++++++++++++++++++++++++++++++
>   2 files changed, 41 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index a4d0059..f55ad01 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 nanoseconds, 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..a027b35 100644
> --- a/drivers/cpuidle/cpuidle.h
> +++ b/drivers/cpuidle/cpuidle.h
> @@ -68,4 +68,42 @@ static inline void cpuidle_coupled_unregister_device(struct cpuidle_device *dev)
>   }
>   #endif
>
> +/*
> + * To ensure that there is no overflow while approximation
> + * for dividing val by 1000, we must respect -
> + * val + (val >> 5) <= 0xFFFFFFFF
> + * val + val/32 <= 0xFFFFFFFF
> + * val <= (0xFFFFFFFF * 32) / 33
> + * val <= 0xF83E0F82
> + * Hence the threshold for val below which we can use the
> + * approximation is 0xF83E0F82
> + */
> +#define DIV_APPROXIMATION_THRESHOLD 0xF83E0F82UL
> +
> +/*
> + * Used for calculating last_residency in usec. Optimized for case
> + * where last_residency in nsecs is < DIV_APPROXIMATION_THRESHOLD
> + * Approximated value has less than 1% error.
> + */
> +static inline int convert_nsec_to_usec(u64 nsec)
> +{
> +	if (likely(nsec < DIV_APPROXIMATION_THRESHOLD)) {
> +		u32 usec = nsec;
> +
> +		usec += usec >> 5;
> +		usec = usec >> 10;
> +
> +		/* Can safely cast to int since usec is < INT_MAX */
> +		return usec;
> +	} else {
> +		u64 usec = div_u64(nsec, 1000);
> +
> +		if (usec > INT_MAX)
> +			usec = INT_MAX;
> +
> +		/* Can safely cast to int since usec is < INT_MAX */
> +		return usec;
> +	}
> +}

What bothers me with this division is the benefit of adding an extra 
ultra optimized division by 1000 in cpuidle.h while we have already 
ktime_divns which is optimized in ktime.h.

Why not:

ts = ns_to_ktime(local_clock());

...

te = ns_to_ktime(local_clock());

diff = ktime_us_delta(te, ts);

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