Re: [RFC][patch 09/12] add xtime_shift and ntp_error_shift to struct timekeeper

john stultz
2009-07-30T22:15:31Z

On Wed, 2009-07-29 at 15:41 +0200, Martin Schwidefsky wrote:
> plain text document attachment (timekeeper-shift.diff)
> From: Martin Schwidefsky <schwidefsky@de.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@elte.hu>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: john stultz <johnstul@us.ibm.com>
> Cc: Daniel Walker <dwalker@fifo99.com>
> Signed-off-by: Martin Schwidefsky <schwidefsky@de.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