netdev - Re: [PATCH] pktgen: Clone skb to avoid corruption of skbs in ndo_start

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Date:	Thu, 21 Jul 2011 19:50:49 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	David Miller <davem@...emloft.net>
Cc:	greearb@...delatech.com, eric.dumazet@...il.com, jpirko@...hat.com,
	netdev@...r.kernel.org, adobriyan@...il.com,
	robert.olsson@....uu.se
Subject: Re: [PATCH] pktgen: Clone skb to avoid corruption of skbs in
 ndo_start_xmit methods

On Thu, Jul 21, 2011 at 03:19:03PM -0700, David Miller wrote:
> From: Ben Greear <greearb@...delatech.com>
> Date: Thu, 21 Jul 2011 15:14:32 -0700
> 
> > On 07/21/2011 03:01 PM, David Miller wrote:
> >> From: Neil Horman<nhorman@...driver.com>
> >> Date: Wed, 20 Jul 2011 11:18:27 -0400
> >>
> >>> On Wed, Jul 20, 2011 at 06:24:15AM +0200, Eric Dumazet wrote:
> >>>> Le mardi 19 juillet 2011 à 22:07 -0400, Neil Horman a écrit :
> >>>>>>
> >>>>> I think this is a good idea.  It lets pktgen dynamically make the
> >>>>> clone/share
> >>>>> decision dynamically and only impacts performance for those systems.
> >>>>>
> >>>>
> >>>> Just let pktgen refuse to use clone_skb command for these devices.
> >>>>
> >>> copy that, This is by no means final, but what do you think of this?
> >>> If its
> >>> agreeable to you, Ben, et al. I can add this to my local tree and
> >>> start auditing
> >>> all the drivers that may need to have the flag set.
> >>
> >> I think there is a much simpler solution.
> >>
> >> Set a flag in the SKB when pktgen does SKB sharing.
> >>
> >> In dev_queue_xmit() (or perhaps, dev_hard_start_xmit()), check the
> >> flag
> >> and if it's set then we copy the SKB.
> >>
> >> If this works, then we fix the crash and no driver changes are
> >> necessary both now and in the future.
> > 
> > Doesn't that make clone-skb in pktgen much less efficient
> > in all cases?
> 
> No, the copy only happens if we enter dev_queue_xmit() which pktgen
> doesn't do, it calls the driver's ->ndo_start_xmit() method directly.
> 
> That's the whole idea.  Only these encapsulating software devices
> will trigger the condition.
> 


I'm happy to go down this route Dave, and agree, its a more solid solution, but
I think the problem with it (which Ben may have been alluding to previously) is
that pktgen doesn't use dev_queue_xmit or dev_hard_start_xmit to send frames.
It mimics the locking of dev_hard_start_xmit (but ignores the other checks that
function does), and just calls the ndo_start_xmit routine of the driver
directly.  So theres no common code that an skb traverses from pktgen to a given
driver where we can check such a flag

Again, I'm happy to change that so that pktgen uses dev_hard_start_xmit, but I
wonder if thats going to get the same sort of pushback about performance that my
origional patch did.  Eric, et al., thoughts?

Regards
Neil

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