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Date: Mon, 06 Jul 2009 11:41:24 -0700
From: john stultz <johnstul@...ibm.com>
To: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] optimized ktime_get[_ts] for GENERIC_TIME=y
On Mon, 2009-07-06 at 15:49 +0200, Martin Schwidefsky wrote:
> From: Martin Schwidefsky <schwidefsky@...ibm.com>
>
> The generic ktime_get function defined in kernel/hrtimer.c is suboptimial
> for GENERIC_TIME=y:
>
> 0) | ktime_get() {
> 0) | ktime_get_ts() {
> 0) | getnstimeofday() {
> 0) | read_tod_clock() {
> 0) 0.601 us | }
> 0) 1.938 us | }
> 0) | set_normalized_timespec() {
> 0) 0.602 us | }
> 0) 4.375 us | }
> 0) 5.523 us | }
>
> Overall there are two read_seqbegin/read_seqretry loops and a lot of
> unnecessary struct timespec calculations. ktime_get returns a nano second
> value which is the sum of xtime, wall_to_monotonic and the nano second
> delta from the clock source.
>
> ktime_get can be optimized for GENERIC_TIME=y. The new version only calls
> clocksource_read:
>
> 0) | ktime_get() {
> 0) | read_tod_clock() {
> 0) 0.610 us | }
> 0) 1.977 us | }
>
> It uses a single read_seqbegin/readseqretry loop and just adds everthing
> to a nano second value.
>
> ktime_get_ts is optimized in a similar fashion.
>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: john stultz <johnstul@...ibm.com>
> Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
Looks good to me.
Acked-by: John Stultz <johnstul@...ibm.com>
> ---
> kernel/hrtimer.c | 4 ++
> kernel/time/timekeeping.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 67 insertions(+)
>
>
> diff -urpN linux-2.5/kernel/hrtimer.c linux-2.5-patched/kernel/hrtimer.c
> --- linux-2.5/kernel/hrtimer.c 2009-07-03 10:46:07.000000000 +0200
> +++ linux-2.5-patched/kernel/hrtimer.c 2009-07-03 10:46:23.000000000 +0200
> @@ -46,6 +46,7 @@
>
> #include <asm/uaccess.h>
>
> +#ifndef CONFIG_GENERIC_TIME
> /**
> * ktime_get - get the monotonic time in ktime_t format
> *
> @@ -60,6 +61,7 @@ ktime_t ktime_get(void)
> return timespec_to_ktime(now);
> }
> EXPORT_SYMBOL_GPL(ktime_get);
> +#endif
>
> /**
> * ktime_get_real - get the real (wall-) time in ktime_t format
> @@ -104,6 +106,7 @@ DEFINE_PER_CPU(struct hrtimer_cpu_base,
> }
> };
>
> +#ifndef CONFIG_GENERIC_TIME
> /**
> * ktime_get_ts - get the monotonic clock in timespec format
> * @ts: pointer to timespec variable
> @@ -128,6 +131,7 @@ void ktime_get_ts(struct timespec *ts)
> ts->tv_nsec + tomono.tv_nsec);
> }
> EXPORT_SYMBOL_GPL(ktime_get_ts);
> +#endif
>
> /*
> * Get the coarse grained time at the softirq based on xtime and
> diff -urpN linux-2.5/kernel/time/timekeeping.c linux-2.5-patched/kernel/time/timekeeping.c
> --- linux-2.5/kernel/time/timekeeping.c 2009-07-03 10:46:07.000000000 +0200
> +++ linux-2.5-patched/kernel/time/timekeeping.c 2009-07-03 10:46:23.000000000 +0200
> @@ -118,6 +118,69 @@ void getnstimeofday(struct timespec *ts)
>
> EXPORT_SYMBOL(getnstimeofday);
>
> +ktime_t ktime_get(void)
> +{
> + cycle_t cycle_now, cycle_delta;
> + struct timespec time;
> + unsigned long seq;
> + s64 nsecs;
> +
> + do {
> + seq = read_seqbegin(&xtime_lock);
> + time.tv_sec = xtime.tv_sec + wall_to_monotonic.tv_sec;
> + time.tv_nsec = xtime.tv_nsec + wall_to_monotonic.tv_nsec;
> +
> + /* read clocksource: */
> + cycle_now = clocksource_read(clock);
> +
> + /* calculate the delta since the last update_wall_time: */
> + cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
> +
> + /* convert to nanoseconds: */
> + nsecs = cyc2ns(clock, cycle_delta);
> +
> + } while (read_seqretry(&xtime_lock, seq));
> + nsecs += time.tv_sec * 1000000000ULL + time.tv_nsec;
> + return (ktime_t) { .tv64 = nsecs };
> +}
> +EXPORT_SYMBOL_GPL(ktime_get);
> +
> +/**
> + * ktime_get_ts - get the monotonic clock in timespec format
> + * @ts: pointer to timespec variable
> + *
> + * The function calculates the monotonic clock from the realtime
> + * clock and the wall_to_monotonic offset and stores the result
> + * in normalized timespec format in the variable pointed to by @ts.
> + */
> +void ktime_get_ts(struct timespec *ts)
> +{
> + cycle_t cycle_now, cycle_delta;
> + struct timespec tomono;
> + unsigned long seq;
> + s64 nsecs;
> +
> + do {
> + seq = read_seqbegin(&xtime_lock);
> + *ts = xtime;
> + tomono = wall_to_monotonic;
> +
> + /* read clocksource: */
> + cycle_now = clocksource_read(clock);
> +
> + /* calculate the delta since the last update_wall_time: */
> + cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
> +
> + /* convert to nanoseconds: */
> + nsecs = cyc2ns(clock, cycle_delta);
> +
> + } while (read_seqretry(&xtime_lock, seq));
> +
> + set_normalized_timespec(ts, ts->tv_sec + tomono.tv_sec,
> + ts->tv_nsec + tomono.tv_nsec + nsecs);
> +}
> +EXPORT_SYMBOL_GPL(ktime_get_ts);
> +
> /**
> * do_gettimeofday - Returns the time of day in a timeval
> * @tv: pointer to the timeval to be set
>
>
--
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