Re: [PATCH 2/10] NTP: normalize time_adj

[john stultz]

On Thu, 2005-12-22 at 00:21 +0100, Roman Zippel wrote:
> This normalizes the calculated value time_adj in second_overflow() to be
> always positive. The difference is added to tick_nsec and stored in
> tick_nsec_curr. This simplifies the work needed in
> update_wall_time_one_tick() as time_phase is always positive.
> 
> Signed-off-by: Roman Zippel <zippel@xxxxxxxxxxxxxx>

Ok, trying to look this code over a bit more closely. 

I really think this area of code needs a lot of work, so I really think
your changes here are cool. That said, this code is difficult to review,
mainly because the existing code is so poorly commented (and probably
also because I'm a bit slow). Not your fault at all, but maybe since
you've grokked it well enough to change it, might you help clarify whats
going on as well? Some (likely incorrect) suggestions below.


> ---
> 
>  include/linux/timex.h |    2 +-
>  kernel/timer.c        |   24 ++++++++++++++----------
>  2 files changed, 15 insertions(+), 11 deletions(-)
> 
> Index: linux-2.6-mm/include/linux/timex.h
> ===================================================================
> --- linux-2.6-mm.orig/include/linux/timex.h	2005-12-21 12:11:48.000000000 +0100
> +++ linux-2.6-mm/include/linux/timex.h	2005-12-21 12:11:56.000000000 +0100
> @@ -93,7 +93,7 @@
>  #define SHIFT_SCALE 22		/* phase scale (shift) */
>  #define SHIFT_UPDATE (SHIFT_KG + MAXTC) /* time offset scale (shift) */
>  #define SHIFT_USEC 16		/* frequency offset scale (shift) */
> -#define FINENSEC (1L << (SHIFT_SCALE - 10)) /* ~1 ns in phase units */
> +#define FINENSEC (1L << SHIFT_SCALE) /* ~1 ns in phase units */

So this effectively increases the granularity of the phase units, right?
Or does the comment need changing?


>  #define MAXPHASE 512000L        /* max phase error (us) */
>  #define MAXFREQ (512L << SHIFT_USEC)  /* max frequency error (ppm) */
> Index: linux-2.6-mm/kernel/timer.c
> ===================================================================
> --- linux-2.6-mm.orig/kernel/timer.c	2005-12-21 12:11:48.000000000 +0100
> +++ linux-2.6-mm/kernel/timer.c	2005-12-21 12:11:56.000000000 +0100
> @@ -552,6 +552,7 @@ found:
>   */
>  unsigned long tick_usec = TICK_USEC; 		/* USER_HZ period (usec) */
>  unsigned long tick_nsec = TICK_NSEC;		/* ACTHZ period (nsec) */
> +static unsigned long tick_nsec_curr = TICK_NSEC;
>  
>  /* 
>   * The current time 
> @@ -601,7 +602,7 @@ long time_next_adjust;
>   */
>  static void second_overflow(void)
>  {
> -	long ltemp;
> +	long ltemp, adj;
>  
>  	/* Bump the maxerror field */
>  	time_maxerror += time_tolerance >> SHIFT_USEC;
> @@ -662,6 +663,7 @@ static void second_overflow(void)
>  		time_state = TIME_OK;
>  	}


Maybe:
	/* reset current nsec_per_tick value for the next tick */
> +	tick_nsec_curr = tick_nsec;
>  	/*
>  	 * Compute the phase adjustment for the next second. In PLL mode, the
>  	 * offset is reduced by a fixed factor times the time constant. In FLL
> @@ -675,36 +677,38 @@ static void second_overflow(void)
>  	ltemp = min(ltemp, (MAXPHASE / MINSEC) << SHIFT_UPDATE);
>  	ltemp = max(ltemp, -(MAXPHASE / MINSEC) << SHIFT_UPDATE);
>  	time_offset -= ltemp;
> -	time_adj = ltemp << (SHIFT_SCALE - SHIFT_HZ - SHIFT_UPDATE);
> +	adj = ltemp << (SHIFT_SCALE - SHIFT_HZ - SHIFT_UPDATE);

	That could maybe use a better comment. The larger comment makes it
clear we're converting the usec offset to phase adjustment units, but
maybe something further to explain how usecs -> phase is connected to
multiplying by 2^(SHIFT_SCALE - SHIFT_HZ - SHIFT_UPDATE) would help?
 
>  	/*
>  	 * Compute the frequency estimate and additional phase adjustment due
>  	 * to frequency error for the next second.
>  	 */
>  	ltemp = time_freq;
> -	time_adj += shift_right(ltemp,(SHIFT_USEC + SHIFT_HZ - SHIFT_SCALE));
> +	adj += shift_right(ltemp,(SHIFT_USEC + SHIFT_HZ - SHIFT_SCALE));

Why not drop ltemp here all together and just use time_freq directly?
Also maybe a brief note about how dividing the shifted ppm frequency by
2^(SHIFT_USEC + SHIFT_HZ - SHIFT_SCALE) converts us to phase units?

>  #if HZ == 100
>  	/*
>  	 * Compensate for (HZ==100) != (1 << SHIFT_HZ).  Add 25% and 3.125% to
>  	 * get 128.125; => only 0.125% error (p. 14)
>  	 */
> -	time_adj += shift_right(time_adj, 2) + shift_right(time_adj, 5);
> +	adj += shift_right(adj, 2) + shift_right(adj, 5);
>  #endif
>  #if HZ == 250
>  	/*
>  	 * Compensate for (HZ==250) != (1 << SHIFT_HZ).  Add 1.5625% and
>  	 * 0.78125% to get 255.85938; => only 0.05% error (p. 14)
>  	 */
> -	time_adj += shift_right(time_adj, 6) + shift_right(time_adj, 7);
> +	adj += shift_right(adj, 6) + shift_right(adj, 7);
>  #endif
>  #if HZ == 1000
>  	/*
>  	 * Compensate for (HZ==1000) != (1 << SHIFT_HZ).  Add 1.5625% and
>  	 * 0.78125% to get 1023.4375; => only 0.05% error (p. 14)
>  	 */
> -	time_adj += shift_right(time_adj, 6) + shift_right(time_adj, 7);
> +	adj += shift_right(adj, 6) + shift_right(adj, 7);
>  #endif
> +	tick_nsec_curr += adj >> (SHIFT_SCALE - 10);
> +	time_adj = (adj << 10) & (FINENSEC - 1);

Again, a comment here would help. 

We use time_adj to increment the phase which will adjust the per-tick
nsec interval in update_wall_time_one_tick, so why are we adjusting
tick_nsec_curr which is used there as well?

Also why SHIFT_SCALE - 10?  And (adj<<10)&(FINSEC -1) looks like magic
to me. :)

>  }
>  
>  /* in the NTP reference this is called "hardclock()" */
> @@ -727,15 +731,15 @@ static void update_wall_time_one_tick(vo
>  		/* Reduce by this step the amount of time left  */
>  		time_adjust -= time_adjust_step;
>  	}
> -	delta_nsec = tick_nsec + time_adjust_step * 1000;
> +	delta_nsec = tick_nsec_curr + time_adjust_step * 1000;
>  	/*
>  	 * Advance the phase, once it gets to one microsecond, then
>  	 * advance the tick more.
>  	 */
>  	time_phase += time_adj;
> -	if ((time_phase >= FINENSEC) || (time_phase <= -FINENSEC)) {
> -		long ltemp = shift_right(time_phase, (SHIFT_SCALE - 10));
> -		time_phase -= ltemp << (SHIFT_SCALE - 10);
> +	if (time_phase >= FINENSEC) {
> +		long ltemp = time_phase >> SHIFT_SCALE;
> +		time_phase -= ltemp << SHIFT_SCALE;
>  		delta_nsec += ltemp;
>  	}
>  	xtime.tv_nsec += delta_nsec;

Again, same point as the last comment.


I'll try to provide similar comments for the other patches tomorrow.

thanks
-john


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