On Mon, Mar 31 2025 at 16:53, Miroslav Lichvar wrote:
> On Thu, Mar 27, 2025 at 04:42:49PM +0100, Miroslav Lichvar wrote:
>> Maybe I could simply patch the kernel to force a small clock
>> multiplier to increase the rate at which the error accumulates.
>
> I tried that and it indeed makes the issue clearly visible. The COARSE
> fix makes the clock less stable. It's barely visible with the normal
> multiplier, at least for the clocksource I tested, but a reduced
> multiplier forces a larger NTP error and raises it above the precision
> and instability of the system and reference clocks.
>
> The test was done on a machine with a TSC clocksource (3GHz CPU with
> disabled frequency scaling - normal multplier is 5592407) and tried a
> multiplier reduced by 4, 16, 64 with this COARSE-fixing patch not
> applied and applied. Each test ran for 1 minute and produced an
> average value of skew - stability of the clock frequency as reported
> by chronyd in the tracking log when synchronizing to a free-running
> PTP clock at 64, 16, and 4 updates per second. It's in parts per
> million (resolution in the chrony log is limited to 0.001 ppm).
>
> Mult reduction Updates/sec Skew before Skew after
> 1 4 0.000 0.000
> 1 16 0.001 0.002
> 1 64 0.002 0.006
> 4 4 0.001 0.001
> 4 16 0.003 0.005
> 4 64 0.005 0.015
> 16 4 0.004 0.009
> 16 16 0.011 0.069
> 16 64 0.020 0.117
> 64 4 0.013 0.012
> 64 16 0.030 0.107
> 64 64 0.058 0.879
Hrm.
Can you try the delta patch below?
Thanks,
tglx
---
kernel/time/timekeeping.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -2234,8 +2234,8 @@ static bool timekeeping_advance(enum tim
tk->tkr_mono.cycle_last, tk->tkr_mono.mask,
tk->tkr_mono.clock->max_raw_delta);
- /* Check if there's really nothing to do */
- if (offset < real_tk->cycle_interval && mode == TK_ADV_TICK)
+ /* Check if there's really something to do */
+ if (offset < real_tk->cycle_interval)
return false;
offset = timekeeping_accumulate(tk, offset, mode, &clock_set);
