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

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Date:	Mon, 18 Nov 2013 10:09:53 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Willy Tarreau" <w@....eu>, "Arnaud Ebalard" <arno@...isbad.org>
Cc:	"Cong Wang" <xiyou.wangcong@...il.com>, <edumazet@...gle.com>,
	<linux-arm-kernel@...ts.infradead.org>, <netdev@...r.kernel.org>,
	"Thomas Petazzoni" <thomas.petazzoni@...e-electrons.com>
Subject: RE: [BUG,REGRESSION?] 3.11.6+,3.12: GbE iface rate drops to few KB/s

> 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



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