netdev - Re: [PATCH] Hyperv: Trigger DHCP renew after host hibernation

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Date:	Sun, 10 Aug 2014 20:51:48 -0700
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Dexuan Cui <decui@...rosoft.com>,
	Greg KH <gregkh@...uxfoundation.org>
CC:	"olaf@...fle.de" <olaf@...fle.de>,
	Richard Weinberger <richard.weinberger@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"driverdev-devel@...uxdriverproject.org" 
	<driverdev-devel@...uxdriverproject.org>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Shao <huishao@...rosoft.com>,
	"Yue Zhang (OSTC DEV)" <yuezha@...rosoft.com>,
	David Miller <davem@...emloft.net>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH] Hyperv: Trigger DHCP renew after host hibernation

Le 10/08/2014 20:23, Dexuan Cui a écrit :
>> -----Original Message-----
>> From: Greg KH [mailto:gregkh@...uxfoundation.org]
>>>>>
>>>>> IMO the most feasible and need-the-least-change solution may be:
>>>>> the hyperv network VSC driver passes the event
>>>>> RNDIS_STATUS_NETWORK_CHANGE to the udev daemon?
>>>>>
>>>> No, don't do that, again, act like any other network device, drop the
>>>> link and bring it up when it comes back.
>>>>
>>> Hi Greg,
>>> Do you mean tearing down the net device and re-creating it (by
>>> register_netdev() and unregister_netdev)?
>>
>> No, don't you have link-detect for your network device?  Toggle that, I
>> thought patches to do this were posted a while ago...
>>
>> But if you really want to tear the whole network device down and then
>> back up again, sure, that would also work.
> Hi Greg, Stephen,
>
> Thanks for the comments!
>
> I suppose you meant the below logic:
> if (refresh) {
>          rtnl_lock();
>          netif_carrier_off(net);
>          netif_carrier_on(net);
>          rtnl_unlock();
> }
>
> We have discussed this in the previous mails of this thread itself:
> e.g., http://marc.info/?l=linux-driver-devel&m=140593811715975&w=2
>
> 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)

Is that such a big deal? If you know you spend much of your time in 
ifplugd, why not use something different that triggers a DHCP renewal 
faster, or fix ifplugd?

>
> If we use this logic in the kernel space, we'll have to "fix" the user-space
> daemons, like ifplugd, systemd-networkd...,etc.

You mean the opposite here don't you? If you put that logic in kernel 
space you don't have to fix the userland.

>
> 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?

That is not acceptable as well, why would an user-space application 
would have to care that much whether it runs on hyper-v or a physical 
host? Not to mention that anytime someone develops a similar but new 
application they would have to become aware of such platform and its 
"specicities".

> BTW2, according to my limited experience, I doubt other VMMs can
> handle this auto-DHCP-renew-in-guest issue properly.
>
> 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.

I think this is just an integration issue that you are having, and I 
would not be focusing on any particular user-space implementation, but 
rather put something in the driver that is sensible, just like what was 
suggested before: toggling the carrier state.

>
> 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 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.
>
> Thanks,
> -- Dexuan
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-- 
Florian
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