netdev - Re: [REGRESSION] r8169: jumbo fixes caused jumbo regressions!

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Date:	Mon, 26 Nov 2012 20:19:51 +0400
From:	Kirill Smelkov <kirr@....spb.ru>
To:	Hayes Wang <hayeswang@...ltek.com>,
	Realtek linux nic maintainers <nic_swsd@...ltek.com>
Cc:	Francois Romieu <romieu@...zoreil.com>,
	"David S. Miller" <davem@...emloft.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	netdev@...r.kernel.org
Subject: Re: [REGRESSION] r8169: jumbo fixes caused jumbo regressions!

On Wed, Nov 14, 2012 at 01:25:30PM +0400, Kirill Smelkov wrote:
> On Tue, Nov 13, 2012 at 11:35:12PM +0100, Francois Romieu wrote:
> > Kirill Smelkov <kirr@....spb.ru> :
> > [...]
> > > My test is to stream raw video from 8 PAL cameras to net - 4 for 720x576@25 and
> > > 4 for 360x288@25 which for YUYV format occupies ~ 860 Mbps of bandwidth. The
> > > program to transmit/receive video is here: http://repo.or.cz/w/rawv.git

[...]
> > > (by the way, on atom system, without tx csum offload, half of cpu time
> > > is spent only to calculate checksums...)
> > 
> > :o(
> 
> yes, that large. In top, my workload is
> 
>                                 %sy     %id     %si
>     
>     default driver load         ~25     ~45     ~27
>     (ethtool -k shows
>      tx-checksumming: off)
> 
>     after                        ~8     ~81     ~11
>     `ethtool -K eth0 tx on`
>      
> 
> that's why the issue is important.
> 
> 
> > > Now I wonder, where that 6K limit came from and why they say it is now
> > > not possible to use jumbos together with tx csum offload ?
> > 
> > Here is an excerpt from a mail where Hayes explained the rules of
> > engagement back in may 2011 (John Lumby and Chris Friesen were Cced then):
> 
> Can't find that mail in gmane netdev archive and on google, to restore
> full context. Was that in private?
> 
> 
> > ! The Max tx sizes for 8168 series are as following:
> > ! 
> > ! 8168B is 4K bytes.
> > ! 8168C and 8168CP are 6K bytes.
> > ! 8168D and later are 9K bytes.
> > ! 
> > ! Note that these sizes all include head size. That is, the mtu must less than
> > ! these values.
> > ! You have to enable Jumbo frame feature when the tx size is large, otherwise the
> > ! packet would not be sent. Because the hw doesn't provide the threshold, the
> > ! checking for MTU > 1500 is just for convenience for sw.
> 
> This part is clear.
> 
> 
> > ! The TSO couldn't work with some feature which need to disable hw checksum, such
> > ! as Jumbo frame. The hw checksum have to be disabled in certain situations, so
> > ! the TSO feature should be checked in these situations, too.
> 
> I don't enable TSO nor I need it. The text indirectly says that hw
> checksum should be disabled when jumbo frames are used.

[...]

> ~~~~
> 
> Hayes, Realtek linux nic maintainers,
> 
>     could you please confirm that for all 8168C and 8168CP jumbo_max is
>     6K and that when jumbos are used, tx checksumming should be off?
> 
>     If so, how come my two chips work stable with ~7K jumbos and tx checksum
>     offload on (tested this night again for ~16 hours without any problem).
> 
>     thanks beforehand.

Dear Hayes, Realtek linux nic maintainers,

Two years ago, for current products, I've specifically choosed
motherboard with RTL8111CP, because Linux driver supported large-enough
Jumbo-frames and tx/rx offload.

Now they say that jumbo-frames should be lowered in length and tx
offload is gone, but my nics still work without problems with old ~7K
jumbos and tx checksum offload. To keep current systems working I either
have to choose another hardware, or patch the driver in contrast to what
people say was the info from the manufacturer.

Neither I like to apply risky patches nor change already proved hardware
to something else without a good reason. So please, as Realtek
representatives,

    could you please confirm that for all 8168C and 8168CP jumbo_max is
    6K and that when jumbos are used, tx checksumming should be off?


Thanks beforehand,
Kirill


P.S. If so, how come my two chips work stable with ~7K jumbos and tx
     checksum offload on (last time tested for ~16 hours without any problem)?
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