From: Simon Guinot

Sent: 13 November 2017 15:36 To: David Miller Cc: thomas.petazzoni@free-electrons.com; netdev@vger.kernel.org; musv@gmx.de; andreas.tobler@cloudguard.ch; gregory.clement@free-electrons.com; antoine.tenart@free-electrons.com; mw@semihalf.com; stable@vger.kernel.org Subject: Re: [PATCH] net: mvneta: fix handling of the Tx descriptor counter

On Mon, Nov 13, 2017 at 11:54:14PM +0900, David Miller wrote:

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From: Simon Guinot simon.guinot@sequanux.org Date: Mon, 13 Nov 2017 15:51:15 +0100

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IIUC the driver stops the queue if a threshold of 316 Tx descriptors is reached (default and worst value).

That's a lot of latency.

OK, then I'll keep the "tx_pending > 255" flushing condition. But note there is no other software mechanism to limit the Tx latency inside the mvneta driver. Should we add something ? And is that not rather the job of the network stack to keep track of the latency and to limit the txq size ?

This is 'first packet transmit latency'.

If the 'doorbell write' is just a PCIe write then, on most systems, that is cheap and pipelined/posted. I'd almost be surprised if you see any 'improvement' from not doing it every packet.

The overall tx queue size is a different issue - usually needs limiting by BQL if TSO is done.

David