[PATCH RFC 0/3] Improve stability of system clock

[Miroslav Lichvar <mlichvar@redhat.com>]

This is an attempt to improve stability and accuracy of the system clock
with very accurate time sources like the new PTP KVM clock or NTP/PTP
using hardware timestamping. It affects mainly kernels running with
NOHZ. It requires updating of the old ia64 and powerpc vsyscalls.

The main problem is that the error accumulated in the ntp_error register
takes too long to correct and this cannot be easily fixed. There are
four sources of the error:
- rounding of time for old vsyscalls
- alignment of frequency adjustments to ticks
- iterative correction of the multiplier
- limited resolution of the multipler

Instead of trying to correct the error faster, the patches remove the
first three sources. With the only remaining source the correction logic
can be simplified and the frequency of the clock is much more stable and
accurate.

Simulations of a frequency step in linux-tktest (values are in ppm and
nanoseconds):

Before:

nohz on             [1, samples/2]           [samples/2 + 1, samples]
samples         freq       dev       max      freq       dev       max
10           1.47222    1341.3    2217.8   0.06322       0.2       0.5
30           0.20799     849.5    2448.7   0.06311       0.2       0.6
100          0.04101     492.1    2895.2   0.06311       0.2       0.5
300          0.05660     295.5    3026.1   0.02064      28.3     108.9
1000         0.01994     409.8    2732.1   0.00355      13.7      52.2
3000         0.00477     469.1    3238.9   0.00070      11.0      40.9
10000        0.00081     377.3    3791.6   0.00013       9.4      36.2
30000        0.00016     259.9    4055.7   0.00004       8.9      34.1
100000       0.00003     159.0    4177.2   0.00000      13.7      58.4

nohz off            [1, samples/2]           [samples/2 + 1, samples]
samples         freq       dev       max      freq       dev       max
10           3.55062       6.2      10.8   0.05730       0.0       0.0
30           0.44672       4.5      14.1   0.05724       0.2       0.5
100          0.03649       2.7      17.4   0.05711       0.2       0.5
300          0.05815       1.7      18.7   0.06313       0.2       0.5
1000         0.06270       1.0      19.1   0.06315       0.2       0.5
3000         0.05720       1.9      19.9   0.02065       1.1       4.1
10000        0.01947      13.5      41.0   0.00339       0.5       1.7
30000        0.00448      17.5      75.9   0.00065       0.3       1.0
100000       0.00078      14.2     101.7   0.00012       0.2       0.7

After:

nohz on             [1, samples/2]           [samples/2 + 1, samples]
samples         freq       dev       max      freq       dev       max
10           0.01584       9.0      14.2   0.02937       2.7       7.2
30           0.00426      10.9      22.4   0.00481       6.5      19.2
100          0.00077      11.6      26.3   0.00074       9.0      26.9
300          0.00013      12.4      29.9   0.00018       8.7      29.3
1000         0.00003      12.6      31.8   0.00003       8.7      32.1
3000         0.00001      12.6      33.3   0.00001       9.1      33.4
10000        0.00000      12.9      34.0   0.00000       9.0      34.1
30000        0.00000      12.8      34.5   0.00000       9.0      34.5
100000       0.00000      16.5      51.2   0.00000      13.7      58.5

nohz off            [1, samples/2]           [samples/2 + 1, samples]
samples         freq       dev       max      freq       dev       max
10           0.10309       0.1       0.1   0.12717       0.0       0.1
30           0.04269       0.1       0.3   0.02592       0.1       0.4
100          0.00629       0.3       0.5   0.00521       0.2       0.5
300          0.00109       0.3       0.6   0.00099       0.2       0.5
1000         0.00019       0.3       0.6   0.00022       0.2       0.6
3000         0.00002       0.3       0.6   0.00002       0.2       0.6
10000        0.00000       0.3       0.6   0.00000       0.2       0.6
30000        0.00000       0.3       0.6   0.00000       0.2       0.6
100000       0.00000       0.3       0.6   0.00000       0.2       0.6

Miroslav Lichvar (3):
  timekeeping: Remove support for old vsyscalls
  timekeeping: Don't align frequency adjustments to ticks
  timekeeping: Determine multiplier directly from NTP tick length

 include/linux/timekeeper_internal.h |   9 +-
 kernel/time/Kconfig                 |   4 -
 kernel/time/timekeeping.c           | 184 +++++++++---------------------------
 3 files changed, 48 insertions(+), 149 deletions(-)

-- 
2.9.3