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c9e6189fb0
Miroslav reported that the periodic RTC synchronization in the NTP code
fails more often than not to hit the specified update window.
The reason is that the code uses delayed_work to schedule the update which
needs to be in thread context as the underlying RTC might be connected via
a slow bus, e.g. I2C. In the update function it verifies whether the
current time is correct vs. the requirements of the underlying RTC.
But delayed_work is using the timer wheel for scheduling which is
inaccurate by design. Depending on the distance to the expiry the wheel
gets less granular to allow batching and to avoid the cascading of the
original timer wheel. See 500462a9de ("timers: Switch to a non-cascading
wheel") and the code for further details.
The code already deals with this by splitting the 660 seconds period into a
long 659 seconds timer and then retrying with a smaller delta.
But looking at the actual granularities of the timer wheel (which depend on
the HZ configuration) the 659 seconds timer ends up in an outer wheel level
and is affected by a worst case granularity of:
HZ Granularity
1000 32s
250 16s
100 40s
So the initial timer can be already off by max 12.5% which is not a big
issue as the period of the sync is defined as ~11 minutes.
The fine grained second attempt schedules to the desired update point with
a timer expiring less than a second from now. Depending on the actual delta
and the HZ setting even the second attempt can end up in outer wheel levels
which have a large enough granularity to make the correctness check fail.
As this is a fundamental property of the timer wheel there is no way to
make this more accurate short of iterating in one jiffies steps towards the
update point.
Switch it to an hrtimer instead which schedules the actual update work. The
hrtimer will expire precisely (max 1 jiffie delay when high resolution
timers are not available). The actual scheduling delay of the work is the
same as before.
The update is triggered from do_adjtimex() which is a bit racy but not much
more racy than it was before:
if (ntp_synced())
queue_delayed_work(system_power_efficient_wq, &sync_work, 0);
which is racy when the work is currently executed and has not managed to
reschedule itself.
This becomes now:
if (ntp_synced() && !hrtimer_is_queued(&sync_hrtimer))
queue_work(system_power_efficient_wq, &sync_work, 0);
which is racy when the hrtimer has expired and the work is currently
executed and has not yet managed to rearm the hrtimer.
Not a big problem as it just schedules work for nothing.
The new implementation has a safe guard in place to catch the case where
the hrtimer is queued on entry to the work function and avoids an extra
update attempt of the RTC that way.
Reported-by: Miroslav Lichvar <mlichvar@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Miroslav Lichvar <mlichvar@redhat.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201206220542.062910520@linutronix.de
164 lines
6.4 KiB
C
164 lines
6.4 KiB
C
/*****************************************************************************
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* *
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* Copyright (c) David L. Mills 1993 *
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* *
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* Permission to use, copy, modify, and distribute this software and its *
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* documentation for any purpose and without fee is hereby granted, provided *
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* that the above copyright notice appears in all copies and that both the *
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* copyright notice and this permission notice appear in supporting *
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* documentation, and that the name University of Delaware not be used in *
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* advertising or publicity pertaining to distribution of the software *
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* without specific, written prior permission. The University of Delaware *
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* makes no representations about the suitability this software for any *
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* purpose. It is provided "as is" without express or implied warranty. *
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* *
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*****************************************************************************/
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/*
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* Modification history timex.h
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*
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* 29 Dec 97 Russell King
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* Moved CLOCK_TICK_RATE, CLOCK_TICK_FACTOR and FINETUNE to asm/timex.h
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* for ARM machines
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*
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* 9 Jan 97 Adrian Sun
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* Shifted LATCH define to allow access to alpha machines.
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*
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* 26 Sep 94 David L. Mills
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* Added defines for hybrid phase/frequency-lock loop.
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*
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* 19 Mar 94 David L. Mills
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* Moved defines from kernel routines to header file and added new
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* defines for PPS phase-lock loop.
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*
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* 20 Feb 94 David L. Mills
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* Revised status codes and structures for external clock and PPS
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* signal discipline.
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*
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* 28 Nov 93 David L. Mills
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* Adjusted parameters to improve stability and increase poll
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* interval.
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*
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* 17 Sep 93 David L. Mills
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* Created file $NTP/include/sys/timex.h
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* 07 Oct 93 Torsten Duwe
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* Derived linux/timex.h
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* 1995-08-13 Torsten Duwe
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* kernel PLL updated to 1994-12-13 specs (rfc-1589)
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* 1997-08-30 Ulrich Windl
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* Added new constant NTP_PHASE_LIMIT
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* 2004-08-12 Christoph Lameter
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* Reworked time interpolation logic
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*/
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#ifndef _LINUX_TIMEX_H
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#define _LINUX_TIMEX_H
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#include <uapi/linux/timex.h>
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#define ADJ_ADJTIME 0x8000 /* switch between adjtime/adjtimex modes */
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#define ADJ_OFFSET_SINGLESHOT 0x0001 /* old-fashioned adjtime */
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#define ADJ_OFFSET_READONLY 0x2000 /* read-only adjtime */
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#include <linux/compiler.h>
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#include <linux/types.h>
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#include <linux/param.h>
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#include <asm/timex.h>
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#ifndef random_get_entropy
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/*
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* The random_get_entropy() function is used by the /dev/random driver
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* in order to extract entropy via the relative unpredictability of
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* when an interrupt takes places versus a high speed, fine-grained
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* timing source or cycle counter. Since it will be occurred on every
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* single interrupt, it must have a very low cost/overhead.
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*
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* By default we use get_cycles() for this purpose, but individual
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* architectures may override this in their asm/timex.h header file.
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*/
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#define random_get_entropy() get_cycles()
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#endif
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/*
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* SHIFT_PLL is used as a dampening factor to define how much we
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* adjust the frequency correction for a given offset in PLL mode.
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* It also used in dampening the offset correction, to define how
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* much of the current value in time_offset we correct for each
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* second. Changing this value changes the stiffness of the ntp
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* adjustment code. A lower value makes it more flexible, reducing
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* NTP convergence time. A higher value makes it stiffer, increasing
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* convergence time, but making the clock more stable.
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*
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* In David Mills' nanokernel reference implementation SHIFT_PLL is 4.
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* However this seems to increase convergence time much too long.
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*
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* https://lists.ntp.org/pipermail/hackers/2008-January/003487.html
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*
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* In the above mailing list discussion, it seems the value of 4
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* was appropriate for other Unix systems with HZ=100, and that
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* SHIFT_PLL should be decreased as HZ increases. However, Linux's
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* clock steering implementation is HZ independent.
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*
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* Through experimentation, a SHIFT_PLL value of 2 was found to allow
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* for fast convergence (very similar to the NTPv3 code used prior to
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* v2.6.19), with good clock stability.
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*
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*
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* SHIFT_FLL is used as a dampening factor to define how much we
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* adjust the frequency correction for a given offset in FLL mode.
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* In David Mills' nanokernel reference implementation SHIFT_FLL is 2.
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*
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* MAXTC establishes the maximum time constant of the PLL.
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*/
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#define SHIFT_PLL 2 /* PLL frequency factor (shift) */
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#define SHIFT_FLL 2 /* FLL frequency factor (shift) */
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#define MAXTC 10 /* maximum time constant (shift) */
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/*
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* SHIFT_USEC defines the scaling (shift) of the time_freq and
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* time_tolerance variables, which represent the current frequency
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* offset and maximum frequency tolerance.
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*/
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#define SHIFT_USEC 16 /* frequency offset scale (shift) */
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#define PPM_SCALE ((s64)NSEC_PER_USEC << (NTP_SCALE_SHIFT - SHIFT_USEC))
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#define PPM_SCALE_INV_SHIFT 19
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#define PPM_SCALE_INV ((1LL << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \
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PPM_SCALE + 1)
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#define MAXPHASE 500000000L /* max phase error (ns) */
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#define MAXFREQ 500000 /* max frequency error (ns/s) */
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#define MAXFREQ_SCALED ((s64)MAXFREQ << NTP_SCALE_SHIFT)
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#define MINSEC 256 /* min interval between updates (s) */
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#define MAXSEC 2048 /* max interval between updates (s) */
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#define NTP_PHASE_LIMIT ((MAXPHASE / NSEC_PER_USEC) << 5) /* beyond max. dispersion */
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/*
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* kernel variables
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* Note: maximum error = NTP synch distance = dispersion + delay / 2;
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* estimated error = NTP dispersion.
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*/
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extern unsigned long tick_usec; /* USER_HZ period (usec) */
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extern unsigned long tick_nsec; /* SHIFTED_HZ period (nsec) */
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/* Required to safely shift negative values */
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#define shift_right(x, s) ({ \
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__typeof__(x) __x = (x); \
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__typeof__(s) __s = (s); \
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__x < 0 ? -(-__x >> __s) : __x >> __s; \
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})
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#define NTP_SCALE_SHIFT 32
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#define NTP_INTERVAL_FREQ (HZ)
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#define NTP_INTERVAL_LENGTH (NSEC_PER_SEC/NTP_INTERVAL_FREQ)
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extern int do_adjtimex(struct __kernel_timex *);
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extern int do_clock_adjtime(const clockid_t which_clock, struct __kernel_timex * ktx);
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extern void hardpps(const struct timespec64 *, const struct timespec64 *);
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int read_current_timer(unsigned long *timer_val);
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/* The clock frequency of the i8253/i8254 PIT */
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#define PIT_TICK_RATE 1193182ul
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#endif /* LINUX_TIMEX_H */
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