netdev - Re: [WIP][PATCHES] Network xmit batching

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Date:	Tue, 03 Jul 2007 09:09:48 -0400
From:	jamal <hadi@...erus.ca>
To:	Matt Carlson <mcarlson@...adcom.com>
Cc:	Robert Olsson <Robert.Olsson@...a.slu.se>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Krishna Kumar2 <krkumar2@...ibm.com>,
	Gagan Arneja <gaagaan@...il.com>, netdev@...r.kernel.org,
	Rick Jones <rick.jones2@...com>,
	Sridhar Samudrala <sri@...ibm.com>,
	David Miller <davem@...emloft.net>,
	Jeff Garzik <jeff@...zik.org>,
	Michael Chan <mchan@...adcom.com>
Subject: Re: [WIP][PATCHES] Network xmit batching - tg3 support

On Mon, 2007-02-07 at 14:20 -0700, Matt Carlson wrote:

> 
> Hi Jamal.  I'll be testing your patch soon,

much thanks. Please let me know if you need help while doing this.
What tools are you planning to test with? I have tested this patch
with pktgen on a dual opteron/tg3-buggy. 
There is an outstanding issue in regards to clocksource - however the
batch does well regardless of the clock source.

>  but I wanted to point out a
> bug in the patch.  The patch defines TG3_SKB_CB() as follows :
> 
> #define TG3_SKB_CB(__skb) ((struct tg3_tx_cbdata *)&((__skb)->cb[0]))
> 
> This definition will collide with the VLAN macros if TG3_VLAN_TAG_USED
> is set.  vlan_tx_tag_get() is defined as :
> 
> #define vlan_tx_tag_get(__skb)  (VLAN_TX_SKB_CB(__skb)->vlan_tag)
> 
> VLAN_TX_SKB_CB is defined as :
> 
> #define VLAN_TX_SKB_CB(__skb) \
>         ((struct vlan_skb_tx_cookie *)&((__skb)->cb[0]))
> 

yikes. Thanks for catching that - I thought i had this pretty much
covered after scanning the source. So that bug exists on the e1000 as
well.
[It sounds very dangerous to me the way skb->cb is being used by the
vlan code (i.e requires human intervention/knowledge to catch it as an
issue). I had no freaking idea the vlan code was using it. Maybe a huge
comment somewhere on how these cbs are being used by drivers would help
or even a registration on startup to just make sure there is no conflict
at a layer (i have been meaning to do the later for years now). In any
case this is is a digression]. 

In the meantime, changing it to use byte 8 and above should do it? i.e.
#define TG3_SKB_CB(__skb) ((struct tg3_tx_cbdata *)&((__skb)->cb[8]))

There are 48 bytes there on the skb-cb, so there should be plenty.

> Also, I think the count, max_per_txd, and nr_frags fields of the
> tg3_tx_cbdata struct are not needed.

Yes, you are right. That was a result of the LinuxWay(tm) (aka cutnpaste
from the e1000 which needs them), Can you send me a patch for that and
TG3_SKB_CB? 

cheers,
jamal

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