Re: [chrony-users] HW Timestamping fails with specific source

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Mon, Feb 12, 2018 at 06:16:02PM +0100, Thibaut BEYLER wrote:
> > Could you please build from git and see if it fixes the issue in your
> > environment?
> 
> Thanks ! back from holiday and tested that. Good news is that TX
> timestamping is now always HW, however RX timestamp is Deamon most of the
> time, sometimes Kernel. However, the general performance are worse now :
> the peer delay is jittery and goes very high. Here's the debug log with my
> two sources (slow :  10.214.11.23 , fast :  10.214.16.11 ) the best one
> having the worst perfs : https://pastebin.com/yi0nYNCm

Thanks. I can reproduce the issue now. It's a race condition in the
kernel and it's not related to HW timestamping.

When chronyd opens a socket with kernel RX timestamping and it's the
only socket on the system using timestamping, it takes some time
before the kernel actually starts timestamping received packets. If a
response is received before that, it will only have a daemon timestamp
and that one is bad due to waiting for the transmit timestamp.

I'm not sure yet how to fix it. We could keep a dummy socket open to
permanently enable timestamping in the kernel, or maybe reuse the
cmdmon sockets for this purpose.

As a workaround, you can enable the NTP server socket, which will keep
the timestamping enabled, e.g. by adding "allow 127.0.0.1" to
chrony.conf.

> > Looking at the logs i can also spots some crazy vales getting 2 secondes
> > > peer delay (and thoses are Deamon/Kernel ) .. mixed timestamp ? Will
> > > investigate more tommorow, thanks
> >
> > Do you have a packet capture or chrony debug output showing this?
> >
> 
> I could not reproduce that issue today but here's a debug log from 2 weeks
> ago that I saved : https://pastebin.com/LUcYsTNz

Interesting. I don't see anything wrong here. It looks more like an
issue with the network or server, and not the timestamping itself, as
there is a >2s gap between the log messages corresponding to the
transmission and reception.

-- 
Miroslav Lichvar

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