Commit 19923c19 authored by Roman Zippel's avatar Roman Zippel Committed by Linus Torvalds
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[PATCH] fix and optimize clock source update



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: default avatarRoman Zippel <zippel@linux-m68k.org>
Acked-by: default avatarJohn Stultz <johnstul@us.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 6415ce9a
......@@ -102,7 +102,7 @@ EXPORT_SYMBOL(tb_ticks_per_sec); /* for cputime_t conversions */
u64 tb_to_xs;
unsigned tb_to_us;
#define TICKLEN_SCALE (SHIFT_SCALE - 10)
#define TICKLEN_SCALE TICK_LENGTH_SHIFT
u64 last_tick_len; /* units are ns / 2^TICKLEN_SCALE */
u64 ticklen_to_xs; /* 0.64 fraction */
......@@ -534,7 +534,7 @@ static __inline__ void timer_recalc_offset(u64 cur_tb)
if (__USE_RTC())
return;
tlen = current_tick_length(SHIFT_SCALE - 10);
tlen = current_tick_length();
offset = cur_tb - do_gtod.varp->tb_orig_stamp;
if (tlen == last_tick_len && offset < 0x80000000u)
return;
......
......@@ -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;
}
......
......@@ -303,8 +303,10 @@ time_interpolator_reset(void)
#endif /* !CONFIG_TIME_INTERPOLATION */
#define TICK_LENGTH_SHIFT 32
/* Returns how long ticks are at present, in ns / 2^(SHIFT_SCALE-10). */
extern u64 current_tick_length(long);
extern u64 current_tick_length(void);
extern int do_adjtimex(struct timex *);
......
......@@ -770,7 +770,7 @@ static void update_ntp_one_tick(void)
* specified number of bits to the right of the binary point.
* This function has no side-effects.
*/
u64 current_tick_length(long shift)
u64 current_tick_length(void)
{
long delta_nsec;
u64 ret;
......@@ -779,14 +779,8 @@ u64 current_tick_length(long shift)
* ie: nanosecond value shifted by (SHIFT_SCALE - 10)
*/
delta_nsec = tick_nsec + adjtime_adjustment() * 1000;
ret = ((u64) delta_nsec << (SHIFT_SCALE - 10)) + time_adj;
/* convert from (SHIFT_SCALE - 10) to specified shift scale: */
shift = shift - (SHIFT_SCALE - 10);
if (shift < 0)
ret >>= -shift;
else
ret <<= shift;
ret = (u64)delta_nsec << TICK_LENGTH_SHIFT;
ret += (s64)time_adj << (TICK_LENGTH_SHIFT - (SHIFT_SCALE - 10));
return ret;
}
......@@ -794,7 +788,6 @@ u64 current_tick_length(long shift)
/* XXX - all of this timekeeping code should be later moved to time.c */
#include <linux/clocksource.h>
static struct clocksource *clock; /* pointer to current clocksource */
static cycle_t last_clock_cycle; /* cycle value at last update_wall_time */
#ifdef CONFIG_GENERIC_TIME
/**
......@@ -813,7 +806,7 @@ static inline s64 __get_nsec_offset(void)
cycle_now = clocksource_read(clock);
/* calculate the delta since the last update_wall_time: */
cycle_delta = (cycle_now - last_clock_cycle) & clock->mask;
cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
/* convert to nanoseconds: */
ns_offset = cyc2ns(clock, cycle_delta);
......@@ -927,7 +920,7 @@ static int change_clocksource(void)
timespec_add_ns(&xtime, nsec);
clock = new;
last_clock_cycle = now;
clock->cycle_last = now;
printk(KERN_INFO "Time: %s clocksource has been installed.\n",
clock->name);
return 1;
......@@ -968,7 +961,7 @@ void __init timekeeping_init(void)
write_seqlock_irqsave(&xtime_lock, flags);
clock = clocksource_get_next();
clocksource_calculate_interval(clock, tick_nsec);
last_clock_cycle = clocksource_read(clock);
clock->cycle_last = clocksource_read(clock);
ntp_clear();
write_sequnlock_irqrestore(&xtime_lock, flags);
}
......@@ -988,7 +981,7 @@ static int timekeeping_resume(struct sys_device *dev)
write_seqlock_irqsave(&xtime_lock, flags);
/* restart the last cycle value */
last_clock_cycle = clocksource_read(clock);
clock->cycle_last = clocksource_read(clock);
write_sequnlock_irqrestore(&xtime_lock, flags);
return 0;
}
......@@ -1014,6 +1007,81 @@ static int __init timekeeping_init_device(void)
device_initcall(timekeeping_init_device);
/*
* If the error is already larger, we look ahead another tick,
* to compensate for late or lost adjustments.
*/
static __always_inline int clocksource_bigadjust(int sign, s64 error, s64 *interval, s64 *offset)
{
int adj;
/*
* As soon as the machine is synchronized to the external time
* source this should be the common case.
*/
error >>= 2;
if (likely(sign > 0 ? error <= *interval : error >= *interval))
return sign;
/*
* An extra look ahead dampens the effect of the current error,
* which can grow quite large with continously late updates, as
* it would dominate the adjustment value and can lead to
* oscillation.
*/
error += current_tick_length() >> (TICK_LENGTH_SHIFT - clock->shift + 1);
error -= clock->xtime_interval >> 1;
adj = 0;
while (1) {
error >>= 1;
if (sign > 0 ? error <= *interval : error >= *interval)
break;
adj++;
}
/*
* Add the current adjustments to the error and take the offset
* into account, the latter can cause the error to be hardly
* reduced at the next tick. Check the error again if there's
* room for another adjustment, thus further reducing the error
* which otherwise had to be corrected at the next update.
*/
error = (error << 1) - *interval + *offset;
if (sign > 0 ? error > *interval : error < *interval)
adj++;
*interval <<= adj;
*offset <<= adj;
return sign << adj;
}
/*
* Adjust the multiplier to reduce the error value,
* this is optimized for the most common adjustments of -1,0,1,
* for other values we can do a bit more work.
*/
static void clocksource_adjust(struct clocksource *clock, s64 offset)
{
s64 error, interval = clock->cycle_interval;
int adj;
error = clock->error >> (TICK_LENGTH_SHIFT - clock->shift - 1);
if (error > interval) {
adj = clocksource_bigadjust(1, error, &interval, &offset);
} else if (error < -interval) {
interval = -interval;
offset = -offset;
adj = clocksource_bigadjust(-1, error, &interval, &offset);
} else
return;
clock->mult += adj;
clock->xtime_interval += interval;
clock->xtime_nsec -= offset;
clock->error -= (interval - offset) << (TICK_LENGTH_SHIFT - clock->shift);
}
/*
* update_wall_time - Uses the current clocksource to increment the wall time
*
......@@ -1021,54 +1089,53 @@ device_initcall(timekeeping_init_device);
*/
static void update_wall_time(void)
{
static s64 remainder_snsecs, error;
s64 snsecs_per_sec;
cycle_t now, offset;
cycle_t offset;
snsecs_per_sec = (s64)NSEC_PER_SEC << clock->shift;
remainder_snsecs += (s64)xtime.tv_nsec << clock->shift;
clock->xtime_nsec += (s64)xtime.tv_nsec << clock->shift;
now = clocksource_read(clock);
offset = (now - last_clock_cycle)&clock->mask;
#ifdef CONFIG_GENERIC_TIME
offset = (clocksource_read(clock) - clock->cycle_last) & clock->mask;
#else
offset = clock->cycle_interval;
#endif
/* normally this loop will run just once, however in the
* case of lost or late ticks, it will accumulate correctly.
*/
while (offset > clock->interval_cycles) {
/* get the ntp interval in clock shifted nanoseconds */
s64 ntp_snsecs = current_tick_length(clock->shift);
while (offset >= clock->cycle_interval) {
/* accumulate one interval */
remainder_snsecs += clock->interval_snsecs;
last_clock_cycle += clock->interval_cycles;
offset -= clock->interval_cycles;
clock->xtime_nsec += clock->xtime_interval;
clock->cycle_last += clock->cycle_interval;
offset -= clock->cycle_interval;
if (clock->xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) {
clock->xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift;
xtime.tv_sec++;
second_overflow();
}
/* interpolator bits */
time_interpolator_update(clock->interval_snsecs
time_interpolator_update(clock->xtime_interval
>> clock->shift);
/* increment the NTP state machine */
update_ntp_one_tick();
/* accumulate error between NTP and clock interval */
error += (ntp_snsecs - (s64)clock->interval_snsecs);
clock->error += current_tick_length();
clock->error -= clock->xtime_interval << (TICK_LENGTH_SHIFT - clock->shift);
}
/* correct the clock when NTP error is too big */
remainder_snsecs += make_ntp_adj(clock, offset, &error);
/* correct the clock when NTP error is too big */
clocksource_adjust(clock, offset);
if (remainder_snsecs >= snsecs_per_sec) {
remainder_snsecs -= snsecs_per_sec;
xtime.tv_sec++;
second_overflow();
}
}
/* store full nanoseconds into xtime */
xtime.tv_nsec = remainder_snsecs >> clock->shift;
remainder_snsecs -= (s64)xtime.tv_nsec << clock->shift;
xtime.tv_nsec = clock->xtime_nsec >> clock->shift;
clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
/* check to see if there is a new clocksource to use */
if (change_clocksource()) {
error = 0;
remainder_snsecs = 0;
clock->error = 0;
clock->xtime_nsec = 0;
clocksource_calculate_interval(clock, tick_nsec);
}
}
......
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