netdev - Re: [PATCH net-next] tc: fix tc actions in case of shared skb

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Message-ID: <55A5B027.70604@gmail.com>
Date:	Tue, 14 Jul 2015 17:58:15 -0700
From:	John Fastabend <john.fastabend@...il.com>
To:	Alexei Starovoitov <ast@...mgrid.com>,
	David Miller <davem@...emloft.net>
CC:	daniel@...earbox.net, jhs@...atatu.com, jiri@...nulli.us,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tc: fix tc actions in case of shared skb

On 15-07-14 04:08 PM, Alexei Starovoitov wrote:
> On 7/14/15 3:34 PM, David Miller wrote:
>>>>> 1 get rid of burst hack for both RX and TX in pktgen (kills
>>>>> >>>performance)
>> #1 is a serious consideration if you don't come up with better ideas,
>> since an optimization is for nothing if it knowingly breaks things.
> 
> I've dug up the pktgen source from 2002 and see:
>   atomic_inc(&skb->users);
>   odev->hard_start_xmit(skb, odev);
> so it did this trick forever.
> Looks like it's a fundamental way how pktgen was working
> and working still. Even when new 'burst' feature is not used,
> pktgen still increments skb->users to hold skb.
> At present I don't have good ideas how to redesign pktgen
> and since apparently no one noticed this tc_action+pktgen
> breakage for years, it's probably ok to leave everything as-is until
> better ideas come. I'm not giving up yet. Just ran out of ideas.
> 

Right and we hit this issue when pktgen is run over any stacked device
with clone_skb set. I've always put it in the don't do this category but
a fix would be nice.

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