linux-kernel - Re: [PATCHv2 0/6] pps: time synchronization over LPT

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Message-ID: <20100323003747.35ea417a@tornado.gnet>
Date:	Tue, 23 Mar 2010 00:37:47 +0300
From:	Alexander Gordeev <lasaine@....cs.msu.su>
To:	john stultz <johnstul@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, linuxpps@...enneenne.com,
	"Nikita V. Youshchenko" <yoush@...msu.su>, stas@....cs.msu.su,
	Rodolfo Giometti <giometti@...eenne.com>
Subject: Re: [PATCHv2 0/6] pps: time synchronization over LPT

В Mon, 22 Mar 2010 14:01:50 -0700
john stultz <johnstul@...ibm.com> пишет:

> On Mon, 2010-03-22 at 23:42 +0300, Alexander Gordeev wrote:
> > Hi John,
> > 
> > Sorry for the delay...
> > 
> > В Mon, 8 Mar 2010 19:25:07 -0800
> > john stultz <johnstul@...ibm.com> пишет:
> > 
> > > On Wed, Feb 24, 2010 at 4:28 AM, Alexander Gordeev
> > > <lasaine@....cs.msu.su> wrote:
> > > > This patchset is tested against the vanilla 2.6.32.9 kernel.
> > > > But we are actually using it on 2.6.31.12-rt20 rt-preempt
> > > > kernel most of the time. Also there is a version which should
> > > > be applied on top of LinuxPPS out of tree patches (i.e. all
> > > > clients and low-level irq timestamps stuff). Those who are
> > > > interested in other versions of the patchset can find them in
> > > > my git repository:
> > > > http://lvk.cs.msu.su/~lasaine/timesync/linux-2.6-timesync.git
> > > >
> > > > There is one problem however: hardpps() works bad when used on
> > > > top of 2.6.33-rc* with CONFIG_NO_HZ enabled. The reason for
> > > > this is commit a092ff0f90cae22b2ac8028ecd2c6f6c1a9e4601.
> > > > Without it hardpps() is able to sync to 1us precision in about
> > > > 10 seconds. With it
> > > 
> > > Uh. Not sure I see right off why the logarithmic time accumulation
> > > would give you troubles. Its actually there to try to fix a
> > > couple of NTP issues that cropped up when the accumulation
> > > interval was pushed out to 2HZ with CONFIG_NO_HZ.
> > 
> > Yes, I know. I guess (based on the commit log and other sources)
> > that this change was added to make chrony work better on tickless
> > kernel. So chrony corrects the time using only frequency
> > corrections?
> 
> I'm not super familiar with chrony, but from talking with folks who
> work on it, it doesn't use the doesn't use the kernel pll, and
> instead uses old oneshot mode. 
> 
> So yea, the logarithmic accumulation did help chrony as well as other
> users of adjtimex that expected to be able to make HZ granular
> adjustments. 
> 
> 
> > My approach is different: use time_offset t remove the phase error
> > and adjust time_freq to remove frequency error.
> 
> This sounds much closer to how NTP does it.
> So are you using the kernel PLL? or just singleshot for the offset?

It's very close a singleshot adjustment, but it uses time_offset instead
of time_adjust. This code does the trick:

static inline s64 ntp_offset_chunk(s64 offset)
{
	if (time_status & STA_PPSTIME && time_status & STA_PPSSIGNAL)
		return offset;
	else
		return shift_right(offset, SHIFT_PLL + time_constant);
}

So when kernel consumer is active, the whole time_offset is applied
immediately.

> > > Do you have any extra insight here as to whats going on with your
> > > code?   The only thing I could guess would be second_overflow() is
> > > happening closer to the actual overflow, but maybe less
> > > regularly? But again, I'm not sure how this would be drastically
> > > different then before with the 2HZ accumulation period.
> > 
> > I still can't find this out (partially because I'm too busy with
> > other tasks). The new code seems ok to mee. time_offset is added at
> > second_overflow as usual. Maybe the problem is with the frequency
> > correction. I'm going to run some tests that should show where the
> > problem is: in the phase or freq correction.
> 
> Yea, you can play with things on the kernel side by setting
> NTP_INTERVAL_FREQ to 2 in include/linux/timex.h to move back to the
> coarser granularity (while preserving the logarithmic accumulation).

Great, thanks for the point! Didn't think of it.

> Do let me know if you find anything here.

Sure!

-- 
  Alexander

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