netdev - Re: pktgen question

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Date:	Mon, 08 Oct 2007 17:46:26 -0500
From:	Steve Wise <swise@...ngridcomputing.com>
To:	David Miller <davem@...emloft.net>
CC:	greearb@...delatech.com, rick.jones2@...com, hadi@...erus.ca,
	johnpol@....mipt.ru, netdev@...r.kernel.org,
	Robert.Olsson@...a.slu.se
Subject: Re: pktgen question



David Miller wrote:
> From: Ben Greear <greearb@...delatech.com>
> Date: Mon, 08 Oct 2007 14:57:13 -0700
> 
>> This skb recycling can certainly work and has been done several
>> times before.  It never gets into the kernel though.
> 
> Because it doesn't work.
> 
> A socket can hang onto a receive packet essentially forever.
> 
> You cannot therefore rely upon the network stack to give you the
> packet back in some reasonable finite amount of time.  This is simply
> the nature of the beast.
> 
> Which means that you either:
> 
> 1) Starve and stop receiving packets when the recycling ring
>    runs out because all of those packets are stuck in socket
>    buffers.  This is easily DoS'able by users on your system
> 
> 2) End up allocating new packets anyways, but then what's the
>    point of the recycling ring?  It's just a hack that works
>    when everything goes as planned, and in real life that is
>    rarely the case.
> 
> It also makes the driver locking more complicated.  Packet
> input occurs in NAPI drivers via netif_receive_skb() which
> would be capable of recycling packets back to the same
> driver in a recycling scheme.  But the recycling can occur
> from other contexts too.  The netif_receive_skb() caller
> already has atomic access to the receive queue, but those
> other callback cases do not.
> 
> That locking issue is just the tip of the iceberg.  Once you
> start discussing solutions, all sorts of new issues begin to
> pop up.
> 
> SKB recycling, just say no.
> 

We're talking about just for pktgen...eh?



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