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Date:	Wed, 13 Aug 2014 15:15:50 +0200
From:	Tom Gundersen <>
To:	Dexuan Cui <>
Cc:	Greg KH <>,
	"" <>,
	Richard Weinberger <>,
	"" <>,
	"" <>,
	Haiyang Zhang <>,
	LKML <>,
	Thomas Shao <>,
	"Yue Zhang (OSTC DEV)" <>,
	David Miller <>,
	Stephen Hemminger <>
Subject: Re: [PATCH] Hyperv: Trigger DHCP renew after host hibernation

On Tue, Aug 12, 2014 at 10:29 AM, Dexuan Cui <> wrote:
>> 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.

Just to avoid any confusion: you are referring to "networkd" (and so
did my comments), but the above logs are from "NetworkManager".

>> 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?  :-)

Sure, but that's clearly not something we should rely on.

>> 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
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