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Date: Thu, 5 Oct 2017 13:05:34 +0200 (CEST) From: Thomas Gleixner <tglx@...utronix.de> To: John Stultz <john.stultz@...aro.org> cc: Gabriel Beddingfield <gabe@...tlabs.com>, LKML <linux-kernel@...r.kernel.org>, Stephen Boyd <sboyd@...eaurora.org>, Alessandro Zummo <a.zummo@...ertech.it>, Alexandre Belloni <alexandre.belloni@...e-electrons.com>, linux-rtc@...r.kernel.org, Guy Erb <guy@...tlabs.com>, hharte@...tlabs.com, Miroslav Lichvar <mlichvar@...hat.com> Subject: Re: Extreme time jitter with suspend/resume cycles On Wed, 4 Oct 2017, John Stultz wrote: > On Wed, Oct 4, 2017 at 9:11 AM, Gabriel Beddingfield <gabe@...tlabs.com> wrote: > > TL;DR: the "delta_delta" hack[1 and 2] in kernel/time/timekeeping.c > > and drivers/rtc/class.c undermines the NTP system. It's not > > appropriate to use if sub-second precision is available. I've attached > > a patch to resolve this... please let me know the ways you hate it. > > :-) > > > > Hello Kernel Timekeeping Maintainers, > > > > We have been developing a device that has very a very aggressive power > > policy, doing suspend/resume cycles a few times a minute ("echo mem > > > /sys/power/state"). In doing so, we found that the system time > > experiences a lot of jitter (compared to, say, an NTP server). It was > > not uncommon for us to see time corrections of 1s to 4s on a regular > > basis. This didn't happen when the device stayed awake, only when it > > was allowed to do suspend/resume. > > > > 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. > > So, on resume when we call __timekeeping_inject_sleeptime(), that uses > the TK_CLEAR_NTP which clears the NTP state (sets STA_UNSYNC, etc) . > I'm not sure how else we can notify userspace. It may be that ntpd > doesn't expect the kernel to set things as unsynced and doesn't > recover well, but the proper fix for that probably is in userspace. Errm. No, __timekeeping_inject_sleeptime() only updates the timekeeper. We have two call sites: timekeeping_resume() { ..... if (sleeptime_injected) __timekeeping_inject_sleeptime(tk, &ts_delta); ... timekeeping_update(tk, TK_MIRROR | TK_CLOCK_WAS_SET); ... } and timekeeping_inject_sleeptime64() { __timekeeping_inject_sleeptime(tk, &delta); ... timekeeping_update(tk, TK_CLEAR_NTP | TK_MIRROR | TK_CLOCK_WAS_SET); ... } But Gabriel talks about the effects from injecting sleep time in timekeeping_resume() because that's where we use read_persistent_clock64(). And there we don't clear NTP, unless there is some magic I'm missing completely. Thanks, tglx
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