[Cake] Possible BUG - parent backlog incorrectly updated in case of NET_XMIT_CN

[Jonathan Morton]

> On 11 Jun, 2016, at 12:11, Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> wrote:
> 
> And now there's a version 2 of Eric's fix: http://marc.info/?l=linux-netdev&m=146507012302643&w=2
> 
> Does this change apply to cake?

I suppose this answers the question of which packet’s size should be used in the correction factor.  Of course, by this time in the enqueue process, the original packet length has been replaced by the overhead-corrected one in Cake, though I can save a copy for this purpose.

And there’s also the problem that we might not need to drop packets as large as the incoming packet in order to fit the latter into the queue - so this corrected correction may be *negative* (the queue is longer than before) - but qdisc_tree_reduce_backlog() only takes an unsigned parameter here.

IMHO the NET_XMIT_CN semantics are broken.  It might be better to drop support for it, since it should rarely be triggered.

 - Jonathan Morton