linux/kernel/time/ntp.c
Miroslav Lichvar 8af3c153ba ntp: Clamp PLL update interval
Clamp update interval to reduce PLL gain with low sampling rate (e.g.
intermittent network connection) to avoid instability.

The clamp roughly corresponds to the loop time constant, it's 8 * poll
interval for SHIFT_PLL 2 and 32 * poll interval for SHIFT_PLL 4. This
gives good results without affecting the gain in normal conditions where
ntpd skips only up to seven consecutive samples.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Acked-by: john stultz <johnstul@us.ibm.com>
LKML-Reference: <1283870626-9472-1-git-send-email-mlichvar@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2010-09-09 20:48:37 +02:00

563 lines
13 KiB
C

/*
* NTP state machine interfaces and logic.
*
* This code was mainly moved from kernel/timer.c and kernel/time.c
* Please see those files for relevant copyright info and historical
* changelogs.
*/
#include <linux/capability.h>
#include <linux/clocksource.h>
#include <linux/workqueue.h>
#include <linux/hrtimer.h>
#include <linux/jiffies.h>
#include <linux/math64.h>
#include <linux/timex.h>
#include <linux/time.h>
#include <linux/mm.h>
/*
* NTP timekeeping variables:
*/
/* USER_HZ period (usecs): */
unsigned long tick_usec = TICK_USEC;
/* ACTHZ period (nsecs): */
unsigned long tick_nsec;
u64 tick_length;
static u64 tick_length_base;
static struct hrtimer leap_timer;
#define MAX_TICKADJ 500LL /* usecs */
#define MAX_TICKADJ_SCALED \
(((MAX_TICKADJ * NSEC_PER_USEC) << NTP_SCALE_SHIFT) / NTP_INTERVAL_FREQ)
/*
* phase-lock loop variables
*/
/*
* clock synchronization status
*
* (TIME_ERROR prevents overwriting the CMOS clock)
*/
static int time_state = TIME_OK;
/* clock status bits: */
int time_status = STA_UNSYNC;
/* TAI offset (secs): */
static long time_tai;
/* time adjustment (nsecs): */
static s64 time_offset;
/* pll time constant: */
static long time_constant = 2;
/* maximum error (usecs): */
static long time_maxerror = NTP_PHASE_LIMIT;
/* estimated error (usecs): */
static long time_esterror = NTP_PHASE_LIMIT;
/* frequency offset (scaled nsecs/secs): */
static s64 time_freq;
/* time at last adjustment (secs): */
static long time_reftime;
static long time_adjust;
/* constant (boot-param configurable) NTP tick adjustment (upscaled) */
static s64 ntp_tick_adj;
/*
* NTP methods:
*/
/*
* Update (tick_length, tick_length_base, tick_nsec), based
* on (tick_usec, ntp_tick_adj, time_freq):
*/
static void ntp_update_frequency(void)
{
u64 second_length;
u64 new_base;
second_length = (u64)(tick_usec * NSEC_PER_USEC * USER_HZ)
<< NTP_SCALE_SHIFT;
second_length += ntp_tick_adj;
second_length += time_freq;
tick_nsec = div_u64(second_length, HZ) >> NTP_SCALE_SHIFT;
new_base = div_u64(second_length, NTP_INTERVAL_FREQ);
/*
* Don't wait for the next second_overflow, apply
* the change to the tick length immediately:
*/
tick_length += new_base - tick_length_base;
tick_length_base = new_base;
}
static inline s64 ntp_update_offset_fll(s64 offset64, long secs)
{
time_status &= ~STA_MODE;
if (secs < MINSEC)
return 0;
if (!(time_status & STA_FLL) && (secs <= MAXSEC))
return 0;
time_status |= STA_MODE;
return div_s64(offset64 << (NTP_SCALE_SHIFT - SHIFT_FLL), secs);
}
static void ntp_update_offset(long offset)
{
s64 freq_adj;
s64 offset64;
long secs;
if (!(time_status & STA_PLL))
return;
if (!(time_status & STA_NANO))
offset *= NSEC_PER_USEC;
/*
* Scale the phase adjustment and
* clamp to the operating range.
*/
offset = min(offset, MAXPHASE);
offset = max(offset, -MAXPHASE);
/*
* Select how the frequency is to be controlled
* and in which mode (PLL or FLL).
*/
secs = get_seconds() - time_reftime;
if (unlikely(time_status & STA_FREQHOLD))
secs = 0;
time_reftime = get_seconds();
offset64 = offset;
freq_adj = ntp_update_offset_fll(offset64, secs);
/*
* Clamp update interval to reduce PLL gain with low
* sampling rate (e.g. intermittent network connection)
* to avoid instability.
*/
if (unlikely(secs > 1 << (SHIFT_PLL + 1 + time_constant)))
secs = 1 << (SHIFT_PLL + 1 + time_constant);
freq_adj += (offset64 * secs) <<
(NTP_SCALE_SHIFT - 2 * (SHIFT_PLL + 2 + time_constant));
freq_adj = min(freq_adj + time_freq, MAXFREQ_SCALED);
time_freq = max(freq_adj, -MAXFREQ_SCALED);
time_offset = div_s64(offset64 << NTP_SCALE_SHIFT, NTP_INTERVAL_FREQ);
}
/**
* ntp_clear - Clears the NTP state variables
*
* Must be called while holding a write on the xtime_lock
*/
void ntp_clear(void)
{
time_adjust = 0; /* stop active adjtime() */
time_status |= STA_UNSYNC;
time_maxerror = NTP_PHASE_LIMIT;
time_esterror = NTP_PHASE_LIMIT;
ntp_update_frequency();
tick_length = tick_length_base;
time_offset = 0;
}
/*
* Leap second processing. If in leap-insert state at the end of the
* day, the system clock is set back one second; if in leap-delete
* state, the system clock is set ahead one second.
*/
static enum hrtimer_restart ntp_leap_second(struct hrtimer *timer)
{
enum hrtimer_restart res = HRTIMER_NORESTART;
write_seqlock(&xtime_lock);
switch (time_state) {
case TIME_OK:
break;
case TIME_INS:
timekeeping_leap_insert(-1);
time_state = TIME_OOP;
printk(KERN_NOTICE
"Clock: inserting leap second 23:59:60 UTC\n");
hrtimer_add_expires_ns(&leap_timer, NSEC_PER_SEC);
res = HRTIMER_RESTART;
break;
case TIME_DEL:
timekeeping_leap_insert(1);
time_tai--;
time_state = TIME_WAIT;
printk(KERN_NOTICE
"Clock: deleting leap second 23:59:59 UTC\n");
break;
case TIME_OOP:
time_tai++;
time_state = TIME_WAIT;
/* fall through */
case TIME_WAIT:
if (!(time_status & (STA_INS | STA_DEL)))
time_state = TIME_OK;
break;
}
write_sequnlock(&xtime_lock);
return res;
}
/*
* this routine handles the overflow of the microsecond field
*
* The tricky bits of code to handle the accurate clock support
* were provided by Dave Mills (Mills@UDEL.EDU) of NTP fame.
* They were originally developed for SUN and DEC kernels.
* All the kudos should go to Dave for this stuff.
*/
void second_overflow(void)
{
s64 delta;
/* Bump the maxerror field */
time_maxerror += MAXFREQ / NSEC_PER_USEC;
if (time_maxerror > NTP_PHASE_LIMIT) {
time_maxerror = NTP_PHASE_LIMIT;
time_status |= STA_UNSYNC;
}
/*
* Compute the phase adjustment for the next second. The offset is
* reduced by a fixed factor times the time constant.
*/
tick_length = tick_length_base;
delta = shift_right(time_offset, SHIFT_PLL + time_constant);
time_offset -= delta;
tick_length += delta;
if (!time_adjust)
return;
if (time_adjust > MAX_TICKADJ) {
time_adjust -= MAX_TICKADJ;
tick_length += MAX_TICKADJ_SCALED;
return;
}
if (time_adjust < -MAX_TICKADJ) {
time_adjust += MAX_TICKADJ;
tick_length -= MAX_TICKADJ_SCALED;
return;
}
tick_length += (s64)(time_adjust * NSEC_PER_USEC / NTP_INTERVAL_FREQ)
<< NTP_SCALE_SHIFT;
time_adjust = 0;
}
#ifdef CONFIG_GENERIC_CMOS_UPDATE
/* Disable the cmos update - used by virtualization and embedded */
int no_sync_cmos_clock __read_mostly;
static void sync_cmos_clock(struct work_struct *work);
static DECLARE_DELAYED_WORK(sync_cmos_work, sync_cmos_clock);
static void sync_cmos_clock(struct work_struct *work)
{
struct timespec now, next;
int fail = 1;
/*
* If we have an externally synchronized Linux clock, then update
* CMOS clock accordingly every ~11 minutes. Set_rtc_mmss() has to be
* called as close as possible to 500 ms before the new second starts.
* This code is run on a timer. If the clock is set, that timer
* may not expire at the correct time. Thus, we adjust...
*/
if (!ntp_synced()) {
/*
* Not synced, exit, do not restart a timer (if one is
* running, let it run out).
*/
return;
}
getnstimeofday(&now);
if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec / 2)
fail = update_persistent_clock(now);
next.tv_nsec = (NSEC_PER_SEC / 2) - now.tv_nsec - (TICK_NSEC / 2);
if (next.tv_nsec <= 0)
next.tv_nsec += NSEC_PER_SEC;
if (!fail)
next.tv_sec = 659;
else
next.tv_sec = 0;
if (next.tv_nsec >= NSEC_PER_SEC) {
next.tv_sec++;
next.tv_nsec -= NSEC_PER_SEC;
}
schedule_delayed_work(&sync_cmos_work, timespec_to_jiffies(&next));
}
static void notify_cmos_timer(void)
{
if (!no_sync_cmos_clock)
schedule_delayed_work(&sync_cmos_work, 0);
}
#else
static inline void notify_cmos_timer(void) { }
#endif
/*
* Start the leap seconds timer:
*/
static inline void ntp_start_leap_timer(struct timespec *ts)
{
long now = ts->tv_sec;
if (time_status & STA_INS) {
time_state = TIME_INS;
now += 86400 - now % 86400;
hrtimer_start(&leap_timer, ktime_set(now, 0), HRTIMER_MODE_ABS);
return;
}
if (time_status & STA_DEL) {
time_state = TIME_DEL;
now += 86400 - (now + 1) % 86400;
hrtimer_start(&leap_timer, ktime_set(now, 0), HRTIMER_MODE_ABS);
}
}
/*
* Propagate a new txc->status value into the NTP state:
*/
static inline void process_adj_status(struct timex *txc, struct timespec *ts)
{
if ((time_status & STA_PLL) && !(txc->status & STA_PLL)) {
time_state = TIME_OK;
time_status = STA_UNSYNC;
}
/*
* If we turn on PLL adjustments then reset the
* reference time to current time.
*/
if (!(time_status & STA_PLL) && (txc->status & STA_PLL))
time_reftime = get_seconds();
/* only set allowed bits */
time_status &= STA_RONLY;
time_status |= txc->status & ~STA_RONLY;
switch (time_state) {
case TIME_OK:
ntp_start_leap_timer(ts);
break;
case TIME_INS:
case TIME_DEL:
time_state = TIME_OK;
ntp_start_leap_timer(ts);
case TIME_WAIT:
if (!(time_status & (STA_INS | STA_DEL)))
time_state = TIME_OK;
break;
case TIME_OOP:
hrtimer_restart(&leap_timer);
break;
}
}
/*
* Called with the xtime lock held, so we can access and modify
* all the global NTP state:
*/
static inline void process_adjtimex_modes(struct timex *txc, struct timespec *ts)
{
if (txc->modes & ADJ_STATUS)
process_adj_status(txc, ts);
if (txc->modes & ADJ_NANO)
time_status |= STA_NANO;
if (txc->modes & ADJ_MICRO)
time_status &= ~STA_NANO;
if (txc->modes & ADJ_FREQUENCY) {
time_freq = txc->freq * PPM_SCALE;
time_freq = min(time_freq, MAXFREQ_SCALED);
time_freq = max(time_freq, -MAXFREQ_SCALED);
}
if (txc->modes & ADJ_MAXERROR)
time_maxerror = txc->maxerror;
if (txc->modes & ADJ_ESTERROR)
time_esterror = txc->esterror;
if (txc->modes & ADJ_TIMECONST) {
time_constant = txc->constant;
if (!(time_status & STA_NANO))
time_constant += 4;
time_constant = min(time_constant, (long)MAXTC);
time_constant = max(time_constant, 0l);
}
if (txc->modes & ADJ_TAI && txc->constant > 0)
time_tai = txc->constant;
if (txc->modes & ADJ_OFFSET)
ntp_update_offset(txc->offset);
if (txc->modes & ADJ_TICK)
tick_usec = txc->tick;
if (txc->modes & (ADJ_TICK|ADJ_FREQUENCY|ADJ_OFFSET))
ntp_update_frequency();
}
/*
* adjtimex mainly allows reading (and writing, if superuser) of
* kernel time-keeping variables. used by xntpd.
*/
int do_adjtimex(struct timex *txc)
{
struct timespec ts;
int result;
/* Validate the data before disabling interrupts */
if (txc->modes & ADJ_ADJTIME) {
/* singleshot must not be used with any other mode bits */
if (!(txc->modes & ADJ_OFFSET_SINGLESHOT))
return -EINVAL;
if (!(txc->modes & ADJ_OFFSET_READONLY) &&
!capable(CAP_SYS_TIME))
return -EPERM;
} else {
/* In order to modify anything, you gotta be super-user! */
if (txc->modes && !capable(CAP_SYS_TIME))
return -EPERM;
/*
* if the quartz is off by more than 10% then
* something is VERY wrong!
*/
if (txc->modes & ADJ_TICK &&
(txc->tick < 900000/USER_HZ ||
txc->tick > 1100000/USER_HZ))
return -EINVAL;
if (txc->modes & ADJ_STATUS && time_state != TIME_OK)
hrtimer_cancel(&leap_timer);
}
getnstimeofday(&ts);
write_seqlock_irq(&xtime_lock);
if (txc->modes & ADJ_ADJTIME) {
long save_adjust = time_adjust;
if (!(txc->modes & ADJ_OFFSET_READONLY)) {
/* adjtime() is independent from ntp_adjtime() */
time_adjust = txc->offset;
ntp_update_frequency();
}
txc->offset = save_adjust;
} else {
/* If there are input parameters, then process them: */
if (txc->modes)
process_adjtimex_modes(txc, &ts);
txc->offset = shift_right(time_offset * NTP_INTERVAL_FREQ,
NTP_SCALE_SHIFT);
if (!(time_status & STA_NANO))
txc->offset /= NSEC_PER_USEC;
}
result = time_state; /* mostly `TIME_OK' */
if (time_status & (STA_UNSYNC|STA_CLOCKERR))
result = TIME_ERROR;
txc->freq = shift_right((time_freq >> PPM_SCALE_INV_SHIFT) *
PPM_SCALE_INV, NTP_SCALE_SHIFT);
txc->maxerror = time_maxerror;
txc->esterror = time_esterror;
txc->status = time_status;
txc->constant = time_constant;
txc->precision = 1;
txc->tolerance = MAXFREQ_SCALED / PPM_SCALE;
txc->tick = tick_usec;
txc->tai = time_tai;
/* PPS is not implemented, so these are zero */
txc->ppsfreq = 0;
txc->jitter = 0;
txc->shift = 0;
txc->stabil = 0;
txc->jitcnt = 0;
txc->calcnt = 0;
txc->errcnt = 0;
txc->stbcnt = 0;
write_sequnlock_irq(&xtime_lock);
txc->time.tv_sec = ts.tv_sec;
txc->time.tv_usec = ts.tv_nsec;
if (!(time_status & STA_NANO))
txc->time.tv_usec /= NSEC_PER_USEC;
notify_cmos_timer();
return result;
}
static int __init ntp_tick_adj_setup(char *str)
{
ntp_tick_adj = simple_strtol(str, NULL, 0);
ntp_tick_adj <<= NTP_SCALE_SHIFT;
return 1;
}
__setup("ntp_tick_adj=", ntp_tick_adj_setup);
void __init ntp_init(void)
{
ntp_clear();
hrtimer_init(&leap_timer, CLOCK_REALTIME, HRTIMER_MODE_ABS);
leap_timer.function = ntp_leap_second;
}