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[PATCH] fix and optimize clock source update

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This fixes the clock source updates in update_wall_time() to correctly
track the time coming in via current_tick_length().  Optimize the fast
paths to be as short as possible to keep the overhead low.

Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
Acked-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Roman Zippel 2006-06-26 00:25:18 -07:00 • committed by Linus Torvalds
parent 6415ce9a92
commit 19923c190e
4 changed files with 124 additions and 150 deletions
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113
include/linux/clocksource.h
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@ -46,8 +46,8 @@ typedef u64 cycle_t;
* @shift: cycle to nanosecond divisor (power of two)
* @update_callback: called when safe to alter clocksource values
* @is_continuous: defines if clocksource is free-running.
* @interval_cycles: Used internally by timekeeping core, please ignore.
* @interval_snsecs: Used internally by timekeeping core, please ignore.
* @cycle_interval: Used internally by timekeeping core, please ignore.
* @xtime_interval: Used internally by timekeeping core, please ignore.
*/
struct clocksource {
char *name;
@ -61,8 +61,9 @@ struct clocksource {
int is_continuous;
/* timekeeping specific data, ignore */
cycle_t interval_cycles;
u64 interval_snsecs;
cycle_t cycle_last, cycle_interval;
u64 xtime_nsec, xtime_interval;
s64 error;
};
/* simplify initialization of mask field */
@ -168,107 +169,11 @@ static inline void clocksource_calculate_interval(struct clocksource *c,
tmp += c->mult/2;
do_div(tmp, c->mult);
c->interval_cycles = (cycle_t)tmp;
if(c->interval_cycles == 0)
c->interval_cycles = 1;
c->cycle_interval = (cycle_t)tmp;
if (c->cycle_interval == 0)
c->cycle_interval = 1;
c->interval_snsecs = (u64)c->interval_cycles * c->mult;
}
/**
* error_aproximation - calculates an error adjustment for a given error
*
* @error: Error value (unsigned)
* @unit: Adjustment unit
*
* For a given error value, this function takes the adjustment unit
* and uses binary approximation to return a power of two adjustment value.
*
* This function is only for use by the the make_ntp_adj() function
* and you must hold a write on the xtime_lock when calling.
*/
static inline int error_aproximation(u64 error, u64 unit)
{
static int saved_adj = 0;
u64 adjusted_unit = unit << saved_adj;
if (error > (adjusted_unit * 2)) {
/* large error, so increment the adjustment factor */
saved_adj++;
} else if (error > adjusted_unit) {
/* just right, don't touch it */
} else if (saved_adj) {
/* small error, so drop the adjustment factor */
saved_adj--;
return 0;
}
return saved_adj;
}
/**
* make_ntp_adj - Adjusts the specified clocksource for a given error
*
* @clock: Pointer to clock to be adjusted
* @cycles_delta: Current unacounted cycle delta
* @error: Pointer to current error value
*
* Returns clock shifted nanosecond adjustment to be applied against
* the accumulated time value (ie: xtime).
*
* If the error value is large enough, this function calulates the
* (power of two) adjustment value, and adjusts the clock's mult and
* interval_snsecs values accordingly.
*
* However, since there may be some unaccumulated cycles, to avoid
* time inconsistencies we must adjust the accumulation value
* accordingly.
*
* This is not very intuitive, so the following proof should help:
* The basic timeofday algorithm: base + cycle * mult
* Thus:
* new_base + cycle * new_mult = old_base + cycle * old_mult
* new_base = old_base + cycle * old_mult - cycle * new_mult
* new_base = old_base + cycle * (old_mult - new_mult)
* new_base - old_base = cycle * (old_mult - new_mult)
* base_delta = cycle * (old_mult - new_mult)
* base_delta = cycle * (mult_delta)
*
* Where mult_delta is the adjustment value made to mult
*
*/
static inline s64 make_ntp_adj(struct clocksource *clock,
cycles_t cycles_delta, s64* error)
{
s64 ret = 0;
if (*error > ((s64)clock->interval_cycles+1)/2) {
/* calculate adjustment value */
int adjustment = error_aproximation(*error,
clock->interval_cycles);
/* adjust clock */
clock->mult += 1 << adjustment;
clock->interval_snsecs += clock->interval_cycles << adjustment;
/* adjust the base and error for the adjustment */
ret = -(cycles_delta << adjustment);
*error -= clock->interval_cycles << adjustment;
/* XXX adj error for cycle_delta offset? */
} else if ((-(*error)) > ((s64)clock->interval_cycles+1)/2) {
/* calculate adjustment value */
int adjustment = error_aproximation(-(*error),
clock->interval_cycles);
/* adjust clock */
clock->mult -= 1 << adjustment;
clock->interval_snsecs -= clock->interval_cycles << adjustment;
/* adjust the base and error for the adjustment */
ret = cycles_delta << adjustment;
*error += clock->interval_cycles << adjustment;
/* XXX adj error for cycle_delta offset? */
}
return ret;
c->xtime_interval = (u64)c->cycle_interval * c->mult;
}