netdev - Re: [PATCH net] bonding: fix arp monitoring with vlan slaves

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Date:	Sat, 03 Aug 2013 02:21:14 +0200
From:	Nikolay Aleksandrov <nikolay@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	andy@...yhouse.net, fubar@...ibm.com
Subject: Re: [PATCH net] bonding: fix arp monitoring with vlan slaves

On 08/03/2013 02:03 AM, Eric Dumazet wrote:
> On Sat, 2013-08-03 at 01:29 +0200, Nikolay Aleksandrov wrote:
> 
>> I knew it was because of the LLTX, but I was wondering about the possible
>> reasons for the xmit_lock_owner check.
>> So basically the arp monitoring (or any dev_trans_start code) won't work
>> with LLTX devices because they don't get their trans_start updated (not the
>> txq trans_start nor the dev->trans_start), is this correct ?
> 
> Nope : LLTX devices are supposed to update their own dev->trans_start
> I added for these case a special comment as in :
> 
> drivers/net/ethernet/atheros/atl1e/atl1e_main.c:1883:   netdev->trans_start = jiffies; /* NETIF_F_LLTX driver :( */
> 
Ah, didn't know about that, so there could be an LLTX device with proper
trans_start if it updates it itself. Fair enough.

> trans_start is a txq property, and vlan devices have a single txq.
> 
> Updating the vlandev->trans_start or the vlantxq->trans_start would be a
> performance killer on MQ ethernet.
> 
Yeah, that part I got, that's why I said it explains a lot for me earlier :-)

> And frankly, as this trans_start is really seldom queried, it makes no
> sense to set it on fast path on the vlan device, if its properly done on
> the real device anyway.
> 
>> But what if the txqs get bound to a particular CPU, then the txq
>> trans_start is okay to be updated I suppose.
> 
> Not sure what you are saying. vlan xmit can be called from any cpus.
> 
Never mind, I didn't notice that the 8021q dev has a single txq as you said.

Thanks for taking the time to explain all this.


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