netdev - Re: [net-next 2/2] macvlan: Enable qdisc backoff logic.

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Date:	Wed, 25 Aug 2010 21:38:00 +0200
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	Ben Greear <greearb@...delatech.com>
Cc:	Arnd Bergmann <arnd@...db.de>, netdev@...r.kernel.org
Subject: Re: [net-next 2/2] macvlan:  Enable qdisc backoff logic.

* Ben Greear | 2010-08-25 12:27:43 [-0700]:

>>I suppose we need to do something in macvtap to handle this as
>>well, right? A guest trying to send a frame through qemu
>>or vhost net into macvtap needs to be prevented from sending
>>more when we get into this path. Right now, we just ignore
>>the return value of macvlan_start_xmit.
>
>I have a similar, though slightly more complex, patch for 802.1q
>vlans, but I haven't looked at macvtap at all.
>
>If these two patches are accepted, I'll post the .1q patch as well.

I do not completely understand the benefit for macvlan. I think this BUSY logic
shifts functionality and make upper level code more complicated (e.g. handle
NET_XMIT_SUCCESS and skb bookkeeping). At the end it boils down to two
scenarios:

a) the congestion is temporary
b) the congestion is for a longer period

For a), a increased link queue length can bridge a longer period too. There is
no need to shift the logic in the upper layer. For b): at the end the upper
layer must also drop skb's - there is no alternative. Or require qemu other,
special handling? (e.g. sleep until the queue is free again).

For case a) the shift in the upper layers _can_ be superior because it can
dynamically increase the skb buffer, whatever. But why not implementing a more
clever, dynamic fifo. E.g. pfifo_dynamic (not really serious)? Is this
functionality qemu centric or are there any other use cases?

Hagen

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