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

Eric Dumazet
2013-07-23T06:10:27Z

On Mon, 2013-07-22 at 20:47 +0100, Ben Hutchings wrote:
> 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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> 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...
>

This sounds a good idea.

Is anybody working on adding SG to usbnet ?

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