Re: [PATCH v2 1/2] time/timekeeping: Fix possible inconsistencies in _COARSE clockids

[Thomas Gleixner <tglx@linutronix.de>]

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);