RE: [PATCH] net: mvneta: fix handling of the Tx descriptor counter

[David Laight <David.Laight@ACULAB.COM>]

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