Re: [PATCH] net: core: explicitly call linkwatch_fire_event to speed up the startup of network services

[Wen Yang <wenyang@linux.alibaba.com>]

在 2020/8/5 上午6:58, David Miller 写道:
> From: Wen Yang <wenyang@linux.alibaba.com>
> Date: Sat,  1 Aug 2020 16:58:45 +0800
> 
>> diff --git a/net/core/link_watch.c b/net/core/link_watch.c
>> index 75431ca..6b9d44b 100644
>> --- a/net/core/link_watch.c
>> +++ b/net/core/link_watch.c
>> @@ -98,6 +98,9 @@ static bool linkwatch_urgent_event(struct net_device *dev)
>>   	if (netif_is_lag_port(dev) || netif_is_lag_master(dev))
>>   		return true;
>>   
>> +	if ((dev->flags & IFF_UP) && dev->operstate == IF_OPER_DOWN)
>> +		return true;
>> +
>>   	return netif_carrier_ok(dev) &&	qdisc_tx_changing(dev);
>>   }
>>   
> 
> You're bypassing explicitly the logic here:
> 
> 	/*
> 	 * Limit the number of linkwatch events to one
> 	 * per second so that a runaway driver does not
> 	 * cause a storm of messages on the netlink
> 	 * socket.  This limit does not apply to up events
> 	 * while the device qdisc is down.
> 	 */
> 	if (!urgent_only)
> 		linkwatch_nextevent = jiffies + HZ;
> 	/* Limit wrap-around effect on delay. */
> 	else if (time_after(linkwatch_nextevent, jiffies + HZ))
> 		linkwatch_nextevent = jiffies;
> 
> Something about this isn't right.  We need to analyze what you are seeing,
> what device you are using, and what systemd is doing to figure out what
> the right place for the fix.
> 
> Thank you.
> 

Thank you very much for your comments.
We are using virtio_net and the environment is a microvm similar to 
firecracker.

Let's briefly explain.
net_device->operstate is assigned through linkwatch_event, and the call 
stack is as follows:
process_one_work
-> linkwatch_event
  -> __linkwatch_run_queue
   -> linkwatch_do_dev
    -> rfc2863_policy
     -> default_operstate

During the machine startup process, net_device->operstate has the 
following two-step state changes:

STEP A: virtnet_probe detects the network card and triggers the 
execution of linkwatch_fire_event.
Since linkwatch_nextevent is initialized to 0, linkwatch_work will run.
And since net_device->state is 6 (__LINK_STATE_PRESENT | 
__LINK_STATE_NOCARRIER), net_device->operstate will be changed from 
IF_OPER_UNKNOWN to IF_OPER_DOWN:
eth0 operstate:0 (IF_OPER_UNKNOWN) -> operstate:2 (IF_OPER_DOWN)

virtnet_probe then executes netif_carrier_on to update 
net_device->state, it will be changed from ‘__LINK_STATE_PRESENT | 
__LINK_STATE_NOCARRIER’ to __LINK_STATE_PRESENT:
eth0 state: 6 (__LINK_STATE_PRESENT | __LINK_STATE_NOCARRIER) -> 2 
(__LINK_STATE_PRESENT)

STEP B: One second later (because linkwatch_nextevent = jiffies + HZ), 
linkwatch_work is executed again.
At this time, since net_device->state is __LINK_STATE_PRESENT, so the 
net_device->operstate will be changed from IF_OPER_DOWN to IF_OPER_UP:
eth0 operstate:2 (IF_OPER_DOWN) -> operstate:6 (IF_OPER_UP)


The above state change can be completed within 2 seconds.
Generally, the machine will load the initramfs first, and do some 
initialization in the initramfs, which takes some time; then switch_root 
to the system disk and continue the initialization, which will also take 
some time, and finally start the systemd-networkd service, bringing 
link, etc.,
In this way, the linkwatch_work work queue has enough time to run twice, 
and the state of net_device->operstate is already IF_OPER_UP,
So bringing link up quickly returns the following information:
Aug 06 16:35:55.966121 iZuf6h1kfgutxc3el68z2lZ systemd-networkd[580]: 
eth0: bringing link up
...
Aug 06 16:35:55.990461 iZuf6h1kfgutxc3el68z2lZ systemd-networkd[580]: 
eth0: flags change: +UP +LOWER_UP +RUNNING

But we are now using MicroVM, which requires extreme speed to start, 
bypassing the initramfs and directly booting the trimmed system on the disk.
systemd-networkd starts in less than 1 second after booting. the STEP B 
has not been run yet, so it will wait for several hundred milliseconds 
here, as follows:
Jul 20 22:00:47.432552 systemd-networkd[210]: eth0: bringing link up
...
Jul 20 22:00:47.446108 systemd-networkd[210]: eth0: flags change: +UP 
+LOWER_UP
...
Jul 20 22:00:47.781463 systemd-networkd[210]: eth0: flags change: +RUNNING


Note: dhcp pays attention to IFF_RUNNING status, we may refer to:
https://www.kernel.org/doc/Documentation/networking/operstates.txt

A routing daemon or dhcp client just needs to care for IFF_RUNNING or
waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before
considering the interface / querying a DHCP address.

Finally, the STEP B above only updates the value of operstate based on 
the known state (operstate/state) on the net_device, without any 
hardware interaction involved, so it is not very reasonable to wait for 
1 second there.

By adding:
+	if ((dev->flags & IFF_UP) && dev->operstate == IF_OPER_DOWN)
+		return true;
+
We hope to improve the linkwatch_urgent_event function a bit.

Hope to get more of your advice and guidance.

Best wishes,
Wen