Messages in this thread | | | Date | Fri, 5 Dec 2014 13:00:22 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [PATCH] optimize ktime_divns for constant divisors |
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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@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 */ > > /* >
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