Subject: Re: [PATCH v2] Provide an interface for getting the current tick
	length
From: john stultz <johnstul@us.ibm.com>
To: Paul Mackerras <paulus@samba.org>
Cc: Roman Zippel <zippel@linux-m68k.org>, torvalds@osdl.org, akpm@osdl.org,
       linux-kernel@vger.kernel.org
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References: <17397.2831.48980.367714@cargo.ozlabs.ibm.com>
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Date: Thu, 16 Feb 2006 16:11:22 -0800
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On Fri, 2006-02-17 at 10:30 +1100, Paul Mackerras wrote:
> This provides an interface for arch code to find out how many
> nanoseconds are going to be added on to xtime by the next call to
> do_timer.  The value returned is a fixed-point number in 52.12 format
> in nanoseconds.  The reason for this format is that it gives the
> full precision that the timekeeping code is using internally.
> 
> The motivation for this is to fix a problem that has arisen on 32-bit
> powerpc in that the value returned by do_gettimeofday drifts apart
> from xtime if NTP is being used.  PowerPC is now using a lockless
> do_gettimeofday based on reading the timebase register and performing
> some simple arithmetic.  (This method of getting the time is also
> exported to userspace via the VDSO.)  However, the factor and offset
> it uses were calculated based on the nominal tick length and weren't
> being adjusted when NTP varied the tick length.
> 
> Note that 64-bit powerpc has had the lockless do_gettimeofday for a
> long time now.  It also had an extremely hairy routine that got called
> from the 32-bit compat routine for adjtimex, which adjusted the
> factor and offset according to what it thought the timekeeping code
> was going to do.  Not only was this only called if a 32-bit task did
> adjtimex (i.e. not if a 64-bit task did adjtimex), it was also
> duplicating computations from kernel/timer.c and it wasn't clear that
> it was (still) correct.
> 
> The simple solution is to ask the timekeeping code how long the
> current jiffy will be on each timer interrupt, after calling
> do_timer.  If this jiffy will be a different length from the last one,
> we then need to compute new values for the factor and offset used in
> the lockless do_gettimeofday.  In this way we can keep xtime and
> do_gettimeofday in sync, even when NTP is varying the tick length.
> 
> Note that when adjtimex varies the tick length, it almost always
> introduces the variation from the next tick on.  The only case I could
> see where adjtimex would vary the length of the current tick is when
> an old-style adjtime adjustment is being cancelled.  (It's not clear
> to me why the adjustment has to be cancelled immediately rather than
> from the next tick on.)  Thus I don't see any real need for a hook in
> adjtimex; the rare case of an old-style adjustment being cancelled can
> be fixed up at the next tick.
> 
> Signed-off-by: Paul Mackerras <paulus@samba.org>
> ---
> This version of the patch has the adjtime calculation factored out and
> the assignments inside if statements removed, as requested.
> 
> diff --git a/include/linux/timex.h b/include/linux/timex.h
> index 04a4a8c..b7ca120 100644
> --- a/include/linux/timex.h
> +++ b/include/linux/timex.h
> @@ -345,6 +345,9 @@ time_interpolator_reset(void)
>  
>  #endif /* !CONFIG_TIME_INTERPOLATION */
>  
> +/* Returns how long ticks are at present, in ns / 2^(SHIFT_SCALE-10). */
> +extern u64 current_tick_length(void);
> +
>  #endif /* KERNEL */
>  
>  #endif /* LINUX_TIMEX_H */
> diff --git a/kernel/timer.c b/kernel/timer.c
> index b9dad39..fe3a9a9 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -717,12 +717,16 @@ static void second_overflow(void)
>  #endif
>  }
>  
> -/* in the NTP reference this is called "hardclock()" */
> -static void update_wall_time_one_tick(void)
> +/*
> + * Returns how many microseconds we need to add to xtime this tick
> + * in doing an adjustment requested with adjtime.
> + */
> +static long adjtime_adjustment(void)
>  {
> -	long time_adjust_step, delta_nsec;
> +	long time_adjust_step;
>  
> -	if ((time_adjust_step = time_adjust) != 0 ) {
> +	time_adjust_step = time_adjust;
> +	if (time_adjust_step) {
>  		/*
>  		 * We are doing an adjtime thing.  Prepare time_adjust_step to
>  		 * be within bounds.  Note that a positive time_adjust means we
> @@ -733,10 +737,19 @@ static void update_wall_time_one_tick(vo
>  		 */
>  		time_adjust_step = min(time_adjust_step, (long)tickadj);
>  		time_adjust_step = max(time_adjust_step, (long)-tickadj);
> +	}
> +	return time_adjust_step;
> +}
>  
> +/* in the NTP reference this is called "hardclock()" */
> +static void update_wall_time_one_tick(void)
> +{
> +	long time_adjust_step, delta_nsec;
> +
> +	time_adjust_step = adjtime_adjustment();
> +	if (time_adjust_step)
>  		/* Reduce by this step the amount of time left  */
>  		time_adjust -= time_adjust_step;

Does the if statement really buy you anything here? 


> -	}
>  	delta_nsec = tick_nsec + time_adjust_step * 1000;
>  	/*
>  	 * Advance the phase, once it gets to one microsecond, then
> @@ -759,6 +772,22 @@ static void update_wall_time_one_tick(vo
>  }
>  
>  /*
> + * Return how long ticks are at the moment, that is, how much time
> + * update_wall_time_one_tick will add to xtime next time we call it
> + * (assuming no calls to do_adjtimex in the meantime).
> + * The return value is in fixed-point nanoseconds with SHIFT_SCALE-10
> + * bits to the right of the binary point.
> + * This function has no side-effects.
> + */
> +u64 current_tick_length(void)
> +{
> +	long delta_nsec;
> +
> +	delta_nsec = tick_nsec + adjtime_adjustment() * 1000;
> +	return ((u64) delta_nsec << (SHIFT_SCALE - 10)) + time_adj;
> +}

You've got time_adj here, but you're not using what's been accumulated
in time_phase, is that really ok?


Other then that it looks ok to me. I know Roman is working on related
code, so I CC'ed him on this.

thanks
-john

-
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