Subject: Re: [PATCH] optimized ktime_get[_ts] for GENERIC_TIME=y
From: john stultz <johnstul@us.ibm.com>
To: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>
In-Reply-To: <20090706154933.5a1f8990@skybase>
References: <20090706154933.5a1f8990@skybase>
Content-Type: text/plain
Date: Mon, 06 Jul 2009 11:41:24 -0700
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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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