linux-kernel - Re: [PATCH 1/1] TX throttling bug-fixing patch of AX88179

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Message-ID: <1374559827.4990.143.camel@edumazet-glaptop>
Date:	Mon, 22 Jul 2013 23:10:27 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.com>,
	Oliver Neukum <oliver@...kum.org>
Cc:	Grant Grundler <grundler@...gle.com>,
	Freddy Xin <freddy@...x.com.tw>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, linux-usb@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	ASIX Louis [蘇威陸] 
	<louis@...x.com.tw>, Allan Chou <allan@...x.com.tw>
Subject: Re: [PATCH 1/1] TX throttling bug-fixing patch of AX88179_178A

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@...il.com> 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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