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

[Ming Lei]

On Tue, Jul 23, 2013 at 2:10 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> 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@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...
>>
>
> This sounds a good idea.
>
> Is anybody working on adding SG to usbnet ?

It depends if size of sg buffer(except for last one) in the sg list can be
divided by usb endpoint's max packet size(512 or 1024), at least there
is the constraint:

http://git.kernel.org/cgit/linux/kernel/git/gregkh/usb.git/commit/?h=usb-next&id=10e232c597ac757e7f8600649f7e872e86de190f

I am wondering if network stack can meet that.  If not, it might be a
bit difficult
because lots of USB host controller don't support that, and driver may have
to support SG and non-SG at the same time for working well on all HCs.

Thanks,
-- 
Ming Lei
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