On Wed, Oct 04, 2017 at 05:16:31PM -0700, John Stultz wrote: > On Wed, Oct 4, 2017 at 9:11 AM, Gabriel Beddingfield <gabe@xxxxxxxxxxxx> wrote: > > We found that the problem is an interaction between the NTP code and > > what I call the "delta_delta hack." (see [1] and [2]) This code > > allocates a static variable in a function that contains an offset from > > the system time to the persistent/rtc clock. It uses that time to > > fudge the suspend timestamp so that on resume the sleep time will be > > compensated. It's kind of a statistical hack that assumes things will > > average out. It seems to have two main assumptions: > > > > 1. The persistent/rtc clock has only single-second precision > > 2. The system does not frequently suspend/resume. > > 3. If delta_delta is less than 2 seconds, these assumptions are "true" > > > > Because the delta_delta hack is trying to maintain an offset from the > > system time to the persistent/rtc clock, any minor NTP corrections > > that have occurred since the last suspend will be discarded. However, > > the NTP subsystem isn't notified that this is happening -- and so it > > causes some level of instability in its PLL logic. This is interesting. What polling interval was ntpd using? If I understand it correctly, with a high-resolution persistent clock the delta-delta compensation should be very small and shouldn't disrupt ntpd. Does this instability disappear when ntpd is not controlling the clock (i.e. "disable ntp" in ntp.conf)? > We should also figure out how to best handle ntpd in userspace dealing > with frequent suspend/resume cycles. This is problematic, as the > closest analogy is trying driving on the road while frequently falling > asleep. This is not something I think ntpd handles well. I suspect > our options are that any ntp adjustments being made might be made for > far too long (causing potentially massive over-correction) or not at > all, and not at all seems slightly better in my mind. Yeah, controlling the clock in such conditions will be difficult. The kernel/ntp PLL requires periodic updates. There is some code in ntp_update_offset() that reduces the frequency adjustment when PLL updates are missing, but I'm not actually sure if it works correctly with suspend. -- Miroslav Lichvar
