[PATCHv2 0/2] Improve stability of system clock

[Miroslav Lichvar <mlichvar@redhat.com>]

v1->v2
- rebased to current code
- improved commit messages

This patch set improves stability and accuracy of the system clock
when synchronized to precise time sources like the new PTP KVM clock or
NTP/PTP with hardware timestamping. The biggest difference is expected
on mostly idle systems running with NOHZ (i.e. having infrequent updates
of the clock).

The stability is affected by the error which accumulates in the
ntp_error register and its correction, which is performed by adjusting
the timekeeping multiplier. After removing the old vsyscalls, which
added an error due to nanosecond rounding, there are now three sources
of the error:
- alignment of frequency adjustments to ticks
- iterative correction of the multiplier
- limited resolution of the multiplier

Instead of trying to correct the error faster, which is difficult to do
without causing instability due to irregular updates of the clock, the
patches remove the first two sources of the error. With the only
remaining source the correction logic can be significantly simplified
and the frequency of the clock is much more stable and accurate.

The freq-step selftest (with PTI disabled to pass the check of the
precision of the MONOTONIC_RAW clock) shows a significant improvement:

Before:

Checking response to frequency step:
  Step           1st interval              2nd interval
             Freq    Dev     Max       Freq    Dev     Max
 40960     +0.131     22     147     +0.136      1       3      [OK]
 40960     +0.012     51     347     +0.005      1       3      [OK]
 40960     +3.832  25038  171297     -0.169      1       2      [OK]
 40960     +2.151  13165   89429     +0.113      1       3      [OK]
 40960     -0.095      1       2     -0.095      1       3      [OK]
   640     -0.154      1       6     -0.154      1       3      [OK]
   640     +0.003      1       6     +0.003      1       3      [OK]
   640     -0.131      2       9     -0.131      1       2      [OK]
   640     -0.026     19     125     -0.029      1       3      [OK]
   640     -0.175      1       3     -0.175      1       3      [OK]
    10     +0.001      3       9     +0.000      3       6      [OK]
    10     +0.002      7      41     +0.000      2       5      [OK]
    10     -0.000      2       5     +0.000      3       5      [OK]
    10     +0.001      5      23     -0.000      2       5      [OK]
    10     +0.000      2       5     -0.000      6      15      [OK]

After:

Checking response to frequency step:
  Step           1st interval              2nd interval
             Freq    Dev     Max       Freq    Dev     Max
 40960     +0.000      2       5     -0.000      2       5      [OK]
 40960     -0.000      2       5     -0.000      3       8      [OK]
 40960     -0.000      3       7     +0.000      3       6      [OK]
 40960     +0.000      3       6     -0.000      2       6      [OK]
 40960     -0.000      2       5     +0.000      3       7      [OK]
   640     +0.001      2       9     -0.000      2       5      [OK]
   640     -0.000      3       6     +0.000      2       5      [OK]
   640     -0.001      2       4     +0.000      1       3      [OK]
   640     -0.000      2       5     +0.000      2       4      [OK]
   640     +0.000      3       6     -0.000      3       6      [OK]
    10     -0.000      2       5     +0.000      2       5      [OK]
    10     -0.000      2       5     +0.000      2       5      [OK]
    10     +0.000      2       5     +0.000      2       5      [OK]
    10     +0.000      2       5     +0.000      2       5      [OK]
    10     +0.000      3      12     +0.000      4      11      [OK]


Miroslav Lichvar (2):
  timekeeping: Don't align frequency adjustments to ticks
  timekeeping: Determine multiplier directly from NTP tick length

 include/linux/timekeeper_internal.h |   2 +
 kernel/time/timekeeping.c           | 140 ++++++++++++------------------------
 2 files changed, 48 insertions(+), 94 deletions(-)

-- 
2.9.5