linux-kernel - Re: Extreme time jitter with suspend/resume cycles

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Date:   Thu, 5 Oct 2017 16:11:09 +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 Thu, 5 Oct 2017, Thomas Gleixner wrote:
> On Wed, 4 Oct 2017, John Stultz wrote:
> > 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.

That should read:

     updates the timekeeper data fields, but does not call
     timekeeping_update().

> 
> 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
> 
> 
>