[PATCH] optimize ktime_divns for constant divisors

[Nicolas Pitre]

BTW this is worth applying despite the on-going discussion with Arnd 
on a separate optimization.

On Wed, 3 Dec 2014, Nicolas Pitre wrote:

> At least on ARM, do_div() is optimized to turn constant divisors into
> an inline multiplication by the reciprocal value at compile time. 
> However this optimization is missed entirely whenever ktime_divns() is
> used and the slow out-of-line division code is used all the time.
> 
> Let ktime_divns() use do_div() inline whenever the divisor is constant
> and small enough.  This will make things like ktime_to_us() and 
> ktime_to_ms() much faster.
> 
> Signed-off-by: Nicolas Pitre <nico at linaro.org>
> 
> diff --git a/include/linux/ktime.h b/include/linux/ktime.h
> index c9d645ad98..411dd8bfe5 100644
> --- a/include/linux/ktime.h
> +++ b/include/linux/ktime.h
> @@ -166,7 +166,17 @@ static inline bool ktime_before(const ktime_t cmp1, const ktime_t cmp2)
>  }
>  
>  #if BITS_PER_LONG < 64
> -extern u64 ktime_divns(const ktime_t kt, s64 div);
> +extern u64 __ktime_divns(const ktime_t kt, s64 div);
> +static inline u64 ktime_divns(const ktime_t kt, s64 div)
> +{
> +	if (__builtin_constant_p(div) && !(div >> 32)) {
> +		u64 ns = kt.tv64;
> +		do_div(ns, div);
> +		return ns;
> +	} else {
> +		return __ktime_divns(kt, div);
> +	}
> +}
>  #else /* BITS_PER_LONG < 64 */
>  # define ktime_divns(kt, div)		(u64)((kt).tv64 / (div))
>  #endif
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 37e50aadd4..890535c41c 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -266,7 +266,7 @@ lock_hrtimer_base(const struct hrtimer *timer, unsigned long *flags)
>  /*
>   * Divide a ktime value by a nanosecond value
>   */
> -u64 ktime_divns(const ktime_t kt, s64 div)
> +u64 __ktime_divns(const ktime_t kt, s64 div)
>  {
>  	u64 dclc;
>  	int sft = 0;
> @@ -282,7 +282,7 @@ u64 ktime_divns(const ktime_t kt, s64 div)
>  
>  	return dclc;
>  }
> -EXPORT_SYMBOL_GPL(ktime_divns);
> +EXPORT_SYMBOL_GPL(__ktime_divns);
>  #endif /* BITS_PER_LONG >= 64 */
>  
>  /*
>