linux-kernel - Re: [RFC][patch 09/12] add xtime_shift and ntp_error

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Date:	Thu, 30 Jul 2009 15:15:23 -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>,
	Daniel Walker <dwalker@...o99.com>
Subject: Re: [RFC][patch 09/12] add xtime_shift and ntp_error_shift to
 struct timekeeper

On Wed, 2009-07-29 at 15:41 +0200, Martin Schwidefsky wrote:
> plain text document attachment (timekeeper-shift.diff)
> From: Martin Schwidefsky <schwidefsky@...ibm.com>
> 
> The xtime_nsec value in the timekeeper structure is shifted by a few
> bits to improve precision. This happens to be the same value as the
> clock->shift. To improve readability add xtime_shift to the timekeeper
> and use it instead of the clock->shift. Likewise add ntp_error_shift
> and replace all (NTP_SCALE_SHIFT - clock->shift) expressions.
> 
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: john stultz <johnstul@...ibm.com>
> Cc: Daniel Walker <dwalker@...o99.com>
> Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
> ---
>  kernel/time/timekeeping.c |   33 ++++++++++++++++++---------------
>  1 file changed, 18 insertions(+), 15 deletions(-)
> 
> Index: linux-2.6/kernel/time/timekeeping.c
> ===================================================================
> --- linux-2.6.orig/kernel/time/timekeeping.c
> +++ linux-2.6/kernel/time/timekeeping.c
> @@ -27,6 +27,8 @@ struct timekeeper {
>  	u32	raw_interval;
>  	u64	xtime_nsec;
>  	s64	ntp_error;
> +	int	xtime_shift;
> +	int	ntp_error_shift;
>  };

I suspect ntp_error_shift is one of the lease intuitive values in the
timekeeper structure. They all probably need nice comments explaining
what they store, but especially this one. 

Something like:

struct clocksource *clock; /* current clocksource used for timekeeping*/
cycle_t cycle_interval;	   /* fixed chunk of cycles used in accumulation */
u64     xtime_interval;    /* number clock shifted nsecs accumulated per cycle_interval */
u32	raw_interval;      /* raw nsecs accumulated per cycle_interval */
u64	xtime_nsec;        /* clock shifted nsecs remainder not stored in xtime.tv_nsec */
s64	ntp_error;         /* Difference between accumulated time and NTP time in ntp shifted nsecs */
int	xtime_shift;       /* The shift value of the current clocksource */
int	ntp_error_shift;   /* Shift conversion between clock shifted nsecs and ntp shifted nsecs */


Other then that this patch looks ok, but will need rigorous testing, as
its very complicated and difficult to understand code thats being
changed.

thanks
-john


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