netdev - RE: [RFC PATCH] net: limit maximum number of packets to mark with xmit

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Date:   Tue, 29 Aug 2017 13:35:49 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jakub Kicinski' <kubakici@...pl>,
        Jacob Keller <jacob.e.keller@...el.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [RFC PATCH] net: limit maximum number of packets to mark with
 xmit_more

From: Jakub Kicinski
> Sent: 25 August 2017 20:34
>
> On Fri, 25 Aug 2017 08:24:49 -0700, Jacob Keller wrote:
> > Under some circumstances, such as with many stacked devices, it is
> > possible that dev_hard_start_xmit will bundle many packets together, and
> > mark them all with xmit_more.
> 
> Excuse my ignorance but what are those stacked devices?  Could they
> perhaps be fixed somehow?  My intuition was that long xmit_more
> sequences can only happen if NIC and/or BQL are back pressuring, and
> therefore we shouldn't be seeing a long xmit_more "train" arriving at
> an empty device ring...

I also suspect that the packets could be coming from multiple sources.
So getting the sources to limit the number of packets with XMIT_MORE
set won't really solve any problem.

At some point the driver for the physical device will have to give it
a kick to start the transmits.

On the systems I've got (desktop x86) PCIe writes aren't really very
expensive.
Reads are a different matter entirely (2us into our fpga target).

	David.