From: Martin Schwidefsky <[email protected]>
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 <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: john stultz <[email protected]>
Cc: Daniel Walker <[email protected]>
Signed-off-by: Martin Schwidefsky <[email protected]>
---
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;
};
struct timekeeper timekeeper;
@@ -66,8 +68,10 @@ static void timekeeper_setup_internals(s
((u64) interval * clock->mult_orig) >> clock->shift;
timekeeper.xtime_nsec = 0;
+ timekeeper.xtime_shift = clock->shift;
timekeeper.ntp_error = 0;
+ timekeeper.ntp_error_shift = NTP_SCALE_SHIFT - clock->shift;
}
/*
@@ -621,8 +625,7 @@ static __always_inline int timekeeping_b
* Now calculate the error in (1 << look_ahead) ticks, but first
* remove the single look ahead already included in the error.
*/
- tick_error = tick_length >>
- (NTP_SCALE_SHIFT - timekeeper.clock->shift + 1);
+ tick_error = tick_length >> (timekeeper.ntp_error_shift + 1);
tick_error -= timekeeper.xtime_interval >> 1;
error = ((error - tick_error) >> look_ahead) + tick_error;
@@ -653,8 +656,7 @@ static void timekeeping_adjust(s64 offse
s64 error, interval = timekeeper.cycle_interval;
int adj;
- error = timekeeper.ntp_error >>
- (NTP_SCALE_SHIFT - timekeeper.clock->shift - 1);
+ error = timekeeper.ntp_error >> (timekeeper.ntp_error_shift - 1);
if (error > interval) {
error >>= 2;
if (likely(error <= interval))
@@ -676,7 +678,7 @@ static void timekeeping_adjust(s64 offse
timekeeper.xtime_interval += interval;
timekeeper.xtime_nsec -= offset;
timekeeper.ntp_error -= (interval - offset) <<
- (NTP_SCALE_SHIFT - timekeeper.clock->shift);
+ timekeeper.ntp_error_shift;
}
/**
@@ -688,7 +690,7 @@ void update_wall_time(void)
{
struct clocksource *clock;
cycle_t offset;
- s64 nsecs;
+ u64 nsecs;
/* Make sure we're fully resumed: */
if (unlikely(timekeeping_suspended))
@@ -700,7 +702,7 @@ void update_wall_time(void)
#else
offset = timekeeper.cycle_interval;
#endif
- timekeeper.xtime_nsec = (s64)xtime.tv_nsec << clock->shift;
+ timekeeper.xtime_nsec = (s64)xtime.tv_nsec << timekeeper.xtime_shift;
/* normally this loop will run just once, however in the
* case of lost or late ticks, it will accumulate correctly.
@@ -711,8 +713,9 @@ void update_wall_time(void)
clock->cycle_last += timekeeper.cycle_interval;
timekeeper.xtime_nsec += timekeeper.xtime_interval;
- if (timekeeper.xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) {
- timekeeper.xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift;
+ nsecs = (u64) NSEC_PER_SEC << timekeeper.xtime_shift;
+ if (timekeeper.xtime_nsec >= nsecs) {
+ timekeeper.xtime_nsec -= nsecs;
xtime.tv_sec++;
second_overflow();
}
@@ -726,7 +729,7 @@ void update_wall_time(void)
/* accumulate error between NTP and clock interval */
timekeeper.ntp_error += tick_length;
timekeeper.ntp_error -= timekeeper.xtime_interval <<
- (NTP_SCALE_SHIFT - clock->shift);
+ timekeeper.ntp_error_shift;
}
/* correct the clock when NTP error is too big */
@@ -751,16 +754,16 @@ void update_wall_time(void)
if (unlikely((s64)timekeeper.xtime_nsec < 0)) {
s64 neg = -(s64)timekeeper.xtime_nsec;
timekeeper.xtime_nsec = 0;
- timekeeper.ntp_error += neg << (NTP_SCALE_SHIFT - clock->shift);
+ timekeeper.ntp_error += neg << timekeeper.ntp_error_shift;
}
/* store full nanoseconds into xtime after rounding it up and
* add the remainder to the error difference.
*/
- xtime.tv_nsec = ((s64)timekeeper.xtime_nsec >> clock->shift) + 1;
- timekeeper.xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
- timekeeper.ntp_error += timekeeper.xtime_nsec <<
- (NTP_SCALE_SHIFT - clock->shift);
+ xtime.tv_nsec = ((s64) timekeeper.xtime_nsec >> timekeeper.xtime_shift) + 1;
+ timekeeper.xtime_nsec -= (s64) xtime.tv_nsec << timekeeper.xtime_shift;
+ timekeeper.ntp_error += timekeeper.xtime_nsec <<
+ timekeeper.ntp_error_shift;
nsecs = clocksource_cyc2ns(offset, clock->mult, clock->shift);
update_xtime_cache(nsecs);
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
On Wed, 2009-07-29 at 15:41 +0200, Martin Schwidefsky wrote:
> plain text document attachment (timekeeper-shift.diff)
> From: Martin Schwidefsky <[email protected]>
>
> 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 <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Cc: john stultz <[email protected]>
> Cc: Daniel Walker <[email protected]>
> Signed-off-by: Martin Schwidefsky <[email protected]>
> ---
> 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
On Thu, 30 Jul 2009 15:15:23 -0700
john stultz <[email protected]> wrote:
> On Wed, 2009-07-29 at 15:41 +0200, Martin Schwidefsky wrote:
> > plain text document attachment (timekeeper-shift.diff)
> > From: Martin Schwidefsky <[email protected]>
> >
> > 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 <[email protected]>
> > Cc: Thomas Gleixner <[email protected]>
> > Cc: john stultz <[email protected]>
> > Cc: Daniel Walker <[email protected]>
> > Signed-off-by: Martin Schwidefsky <[email protected]>
> > ---
> > 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 */
For xtime_shift I would like to use a different comment. As I think
about the value it is the number of bits the xtime_nsec value is
shifted to improve precision. It is the same as clock->shift but the
semantics is different. How about:
/* Shift conversion between the xtime_nsec remainder and nsecs */
int xtime_shift;
The other comments are good, I've added them.
> Other then that this patch looks ok, but will need rigorous testing, as
> its very complicated and difficult to understand code thats being
> changed.
This patch in particular is rather easy to verify, but the overall series
will definitely need some time in linux-next.
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.