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    SubjectRe: [PATCH] optimized ktime_get[_ts] for GENERIC_TIME=y
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    On Mon, 2009-07-06 at 15:49 +0200, Martin Schwidefsky wrote:
    > From: Martin Schwidefsky <schwidefsky@de.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@elte.hu>
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Cc: john stultz <johnstul@us.ibm.com>
    > Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>


    Looks good to me.
    Acked-by: John Stultz <johnstul@us.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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