Re: [PATCH 1/1] TX throttling bug-fixing patch of AX88179_178A

[Ben Hutchings]

On Mon, 2013-07-22 at 11:47 -0700, Eric Dumazet wrote:
> On Mon, 2013-07-22 at 19:38 +0100, Ben Hutchings wrote:
> > On Mon, 2013-07-22 at 11:29 -0700, Grant Grundler wrote:
> > > On Mon, Jul 22, 2013 at 10:07 AM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> > > ...
> > > > I guess that if a driver does not advertise NETIF_F_SG, this
> > > > skb_linearize() call is not needed : All frames reaching your xmit
> > > > function should already be linear
> > > 
> > > As Ben Hutchings pointed out, hw_features is still setting this...but
> > > I'm not sure how that matters.
> > > 
> > > ax88179_set_features() doesn't allow setting SG or TSO features.  But
> > > I expect it would be "not too difficult" to add such that ethtool
> > > could set those features after boot.
> > [...]
> > 
> > It already can.  That's what putting feature flags in hw_features does.
> 
> My original concern, that inspired this patch, was to remove SG support,
> as this driver does not have SG support at all.
> 
> Linearize a full TSO packet needs order-5 allocations, thats likely to
> fail and lead to very slow TCP performance, because it will only rely on
> retransmits.

The driver could set gso_max_size to reduce that problem.  But I rather
doubt that TSO followed by skb_linearize() significantly improves
throughput or CPU-efficiency.  (If the device has a 1G link but is
connected to the host through a USB 2.0 port, then USB is the bottleneck
and TSO could improve throughput a few percent.  But that's a silly
configuration.)

The real solution would be for someone to add SG support to the usbnet
core.  Trying to support 1GbE with only linear skbs is not a great
idea... and it can only be a matter of time before there is USB ultra
speed (or whatever comes after 'super') with 10GbE devices...

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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