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ktime: Optimize ktime_divns for constant divisors

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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.

Cc: Arnd Bergmann <arnd.bergmann@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Nicolas Pitre <nico@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
This commit is contained in:
Nicolas Pitre 2014-12-03 14:43:06 -05:00 committed by John Stultz
parent eaa27f34e9
commit 8b618628b2
2 changed files with 13 additions and 3 deletions
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12
include/linux/ktime.h
View File

@ -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

4
kernel/time/hrtimer.c
View File

@ -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 */
/*