RE: [PATCH] Hyperv: Trigger DHCP renew after host hibernation

[Dexuan Cui]

> From: Tom Gundersen
> > Unluckily this logic doesn't work because the user-space daemons
> > like ifplugd, usually don't renew the DHCP immediately as long as they
> > receive a link-down message: they usually wait for some seconds and if
> > they find the link becomes up soon, they won't trigger renew operations.
> > (I guess this behavior can be somewhat reasonable: maybe the daemons
> > try to not trigger DHCP renew on temporary link instability)
> >
> networkd does not suffer from this problem, and in ifplugd it can be
I didn't have time to check the code of networkd, but I think it does have the
same behavior.
e.g., on a bare metal host with Ubuntu 14.04, when I plug the RJ45 cable out
of the network card and then plug the cable back into the network card
quickly -- in ~3 seconds, networkd doesn't trigger DHCP renew request: in
/var/log/syslog, we see
Aug 12 11:07:07 decui-lin NetworkManager[828]: <info> (eth0): carrier now OFF (device state 100, deferring action for 4 seconds)
Aug 12 11:07:07 decui-lin kernel: [  246.975453] e1000e: eth0 NIC Link is Down
Aug 12 11:07:10 decui-lin NetworkManager[828]: <info> (eth0): carrier now ON (device state 100)
Aug 12 11:07:10 decui-lin kernel: [  250.028533] e1000e: eth0 NIC Link is Up 100 Mbps Full Duplex, Flow Control: Rx/Tx

It looks there is a delay of 4s.
I'm going to find out if there is a configurable parameter for this.

> disabled. Most other network drivers will send
> IFF_LOWER_DOWN/IFF_LOWER_UP upon suspend/resume so if you were to
> do the same you will not work any worse than the others. What would be
suspend/resume(ACPI S3?) is different  as this is usually > 4 seconds?  :-)

> nice, as mentioned by Dan and Lennart, would be to send an additional
> explicit event such as "resumed from suspend" or "L3 may be wrong".
Sorry, I neglected to reply this.
IMHO even if we add the new event, we still need lots of efforts to
make the various userland daemons(ifplugd, networkd, etc) to use the
new event.
And looks we're the first user of this new event. I'm not sure if this issue
here can convince the network subsystem maintainers such a new event
is a must.

> That should be a generic thing though, to fix all such issues in one
> go.
I agree, though this requires we update all the userland daemons...
 
> > I'm not sure our attempt to "fix" the daemons can be easily accepted.
> > BTW, by CPUID, an application has a reliable way to determine  if it's
> > running on hyper-v on not. Maybe we can "fix" the behavior of the
> > daemons when they run on hyper-v?
> > BTW2, according to my limited experience, I doubt other VMMs can
> > handle this auto-DHCP-renew-in-guest issue properly.
> 
> To the extent this is a problem, it is a generic one, so we should not
> need any hyper-v specific logic in userspace.
OK, I understood.
 
> > That was why Yue's patch wanted to add a SLEEP(10s) between the
> > link-down and link-up events and hoped this could be an acceptable
> > fix(while it turned out not, obviously), though we admit it's not so good
> > to add such a magic number "10s" in a kernel driver.
> >
> > Please point it out if I missed or misunderstand something.
> >
> > Now I understand it's not good to pass the event to the udev daemon,
> > and it's not good to use a SLEEP(10s) in the kernel space(even if it's in a
> > "work" task here).
> 
> Please just expose to userspace what is happening (link lost/gained,
> resumed from suspend/...), and let us sort out how to react to that.
> If you put assumptions about what kind of timeouts (long-dead)
> userspace uses into your drivers you'll just create a mess.
OK, I got it now.
So I think I'm supposed to send out a netif_carrier_off()/on() patch,
and I'd better do this after I verify the daemons can work with
proper parameters specified.

> > Please let me know if it's the correct direction to fix the user-space
> > daemons (ifplugd, systemd-networkd, etc).
> > If you think this is viable and we should do this, I'll submit a
> > netif_carrier_off/on patch first and will start to work with the
> > projects of ifplugd, systemd-networkd and many OSVs to make the
> > while thing work eventually.
> 
> Have you actually checked that carrier_off/on does not work on
> anything but ifplugd? It would surprise me...
I can confirm carrier_off/on with 0 delay between the off and on
doesn't work for ifplugd and networkd.

Thanks,
-- Dexuan