Re: [PATCH] optimize ktime_divns for constant divisors

From: Nicolas Pitre
Date: Fri Dec 05 2014 - 13:00:30 EST


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@xxxxxxxxxx>
>
> 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 */
>
> /*
>
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