[BUG,REGRESSION?] 3.11.6+,3.12: GbE iface rate drops to few KB/s

[David Laight]

> I wonder if we can call mvneta_txq_done() from the IRQ handler, which would
> cause some recycling of the Tx descriptors when receiving the corresponding
> ACKs.
> 
> Ideally we should enable the Tx IRQ, but I still have no access to this
> chip's datasheet despite having asked Marvell several times in one year
> (Thomas has it though).
> 
> So it is fairly possible that in your case you can't fill the link if you
> consume too many descriptors. For example, if your server uses TCP_NODELAY
> and sends incomplete segments (which is quite common), it's very easy to
> run out of descriptors before the link is full.

Or you have a significant number of active tcp connections.

Even if there were no requirement to free the skb quickly you still
need to take a 'tx done' interrupt when the link is transmit rate limited.
There are scenarios when there is no receive traffic - eg asymmetric
routing, but testable with netperf UDP transmits.

	David