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sys_generic: damp slew oscillation due to delayed stop

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If the computer is overloaded so much that chronyd cannot stop a slew
within one second of the scheduled end and the actual duration is more
than doubled (2 seconds with the minimum duration of 1 second), the
overshoot will be larger than the intended correction. If these
conditions persist, the oscillation will grow up to the maximum offset
allowed by maxslewrate and the delay in stopping.

Monitor the excess duration as an exponentially decaying maximum value
and don't allow any slews shorter than 5 times the value to damp the
oscillation. Ignore delays longer than 100 seconds, assuming they have a
different cause (e.g. the system was suspended and resumed) and are
already handled in the scheduler by triggering cancellation of the
ongoing slew.

This should also make it safer to shorten the minimum duration if
needed.

Reported-by: Daniel Franke <dff@amazon.com>
This commit is contained in:
Miroslav Lichvar 2022-06-14 16:31:22 +02:00
parent af8e4a5115
commit 2ed88c31c7
1 changed files with 35 additions and 5 deletions
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40
sys_generic.c
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@ -80,6 +80,21 @@ static struct timespec slew_start;
/* Scheduler timeout ID for ending of the currently running slew */
static SCH_TimeoutID slew_timeout_id;
/* Scheduled duration of the currently running slew */
static double slew_duration;
/* Expected delay in ending of the slew due to process scheduling and
execution time, tracked as a decaying maximum value */
static double slew_excess_duration;
/* Maximum accepted excess duration to ignore large jumps after resuming
suspended system and other reasons (which should be handled in the
scheduler), a constant to determine the minimum slew duration to avoid
oscillations due to the excess, and the decay constant */
#define MAX_SLEW_EXCESS_DURATION 100.0
#define MIN_SLEW_DURATION_EXCESS_RATIO 5.0
#define SLEW_EXCESS_DURATION_DECAY 0.9
/* Suggested offset correction rate (correction time * offset) */
static double correction_rate;
@ -164,8 +179,8 @@ clamp_freq(double freq)
static void
update_slew(void)
{
double old_slew_freq, total_freq, corr_freq, duration, excess_duration;
struct timespec now, end_of_slew;
double old_slew_freq, total_freq, corr_freq, duration;
/* Remove currently running timeout */
SCH_RemoveTimeout(slew_timeout_id);
@ -178,13 +193,25 @@ update_slew(void)
stop_fastslew(&now);
/* Estimate how long should the next slew take */
/* Update the maximum excess duration, decaying even when the slew did
not time out (i.e. frequency was set or offset accrued), but add a small
value to avoid denormals */
slew_excess_duration = (slew_excess_duration + 1.0e-9) * SLEW_EXCESS_DURATION_DECAY;
excess_duration = duration - slew_duration;
if (slew_excess_duration < excess_duration &&
excess_duration <= MAX_SLEW_EXCESS_DURATION)
slew_excess_duration = excess_duration;
/* Calculate the duration of the new slew, considering the current correction
rate and previous delays in stopping of the slew */
if (fabs(offset_register) < MIN_OFFSET_CORRECTION) {
duration = MAX_SLEW_DURATION;
} else {
duration = correction_rate / fabs(offset_register);
if (duration < MIN_SLEW_DURATION)
duration = MIN_SLEW_DURATION;
if (duration < MIN_SLEW_DURATION_EXCESS_RATIO * slew_excess_duration)
duration = MIN_SLEW_DURATION_EXCESS_RATIO * slew_excess_duration;
}
/* Get frequency offset needed to slew the offset in the duration
@ -240,10 +267,12 @@ update_slew(void)
UTI_AddDoubleToTimespec(&now, duration, &end_of_slew);
slew_timeout_id = SCH_AddTimeout(&end_of_slew, handle_end_of_slew, NULL);
slew_start = now;
slew_duration = duration;
DEBUG_LOG("slew offset=%e corr_rate=%e base_freq=%f total_freq=%f slew_freq=%e duration=%f slew_error=%e",
offset_register, correction_rate, base_freq, total_freq, slew_freq,
duration, slew_error);
DEBUG_LOG("slew offset=%e corr_rate=%e base_freq=%f total_freq=%f slew_freq=%e"
" duration=%f excess=%f slew_error=%e",
offset_register, correction_rate, base_freq, total_freq, slew_freq,
slew_duration, slew_excess_duration, slew_error);
}
/* ================================================== */
@ -380,6 +409,7 @@ SYS_Generic_CompleteFreqDriver(double max_set_freq_ppm, double max_set_freq_dela
base_freq = (*drv_read_freq)();
slew_freq = 0.0;
offset_register = 0.0;
slew_excess_duration = 0.0;
max_corr_freq = CNF_GetMaxSlewRate() / 1.0e6;