raw_skew measures the drift between CLOCK_MONOTONIC and CLOCK_MONOTONIC_RAW and compares it against the adjustment adjtimex() reports, but for the latter it reads only tx.freq. The kernel takes the tick value into the adjustment as well, worth a microsecond of the tick length per unit, a hundred ppm each, and a time synchronisation daemon does put the coarse part of its correction there. Where it does, the two numbers cannot meet:
# Estimating clock drift: -51.724(est) 48.277(act) [FAILED] On the machine measured, chronyd held freq at 48.278 ppm with tick at 9999, so the correction really applied is -51.722 ppm, which is what the clocks show. The skip for an externally adjusted clock does not catch this: the clock is steady, the offset is zero, and neither freq nor tick moves during the run. Count what tick is worth alongside freq. On the same machine: # Estimating clock drift: -51.724(est) -51.723(act) [OK] Signed-off-by: Eva Kurchatova <[email protected]> --- tools/testing/selftests/timers/raw_skew.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/tools/testing/selftests/timers/raw_skew.c b/tools/testing/selftests/timers/raw_skew.c index 0c87a8fb0d7f..dfca3e2d50d2 100644 --- a/tools/testing/selftests/timers/raw_skew.c +++ b/tools/testing/selftests/timers/raw_skew.c @@ -91,6 +91,7 @@ int main(int argc, char **argv) { struct timespec mon, raw, start, end; long long delta1, delta2, interval, eppm, ppm; + long tick_nom; struct timex tx1, tx2; setbuf(stdout, NULL); @@ -129,6 +130,17 @@ int main(int argc, char **argv) /* Avg the two actual freq samples adjtimex gave us */ ppm = (long long)(tx1.freq + tx2.freq) * 1000 / 2; ppm = shift_right(ppm, 16); + + /* + * The tick value holds the coarse part of the adjustment, a + * microsecond of the tick length per unit, and time sync daemons + * do put part of their correction there. What it is worth has to + * be counted in as well, or a clock disciplined through it looks + * off by a hundred ppm a unit against what the two clocks show. + */ + tick_nom = USEC_PER_SEC / sysconf(_SC_CLK_TCK); + ppm += (long long)(tx1.tick + tx2.tick - 2 * tick_nom) * + (USEC_PER_SEC / tick_nom) * 1000 / 2; printf(" %lld.%i(act)", ppm/1000, abs((int)(ppm%1000))); if (llabs(eppm - ppm) > 1000) { -- 2.55.0
