netdev - Re: Initial thoughts on TXDP

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Date:   Fri, 2 Dec 2016 14:36:33 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Tom Herbert <tom@...bertland.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>
CC:     Florian Westphal <fw@...len.de>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>
Subject: Re: Initial thoughts on TXDP

On 01/12/16 23:46, Tom Herbert wrote:
> The only time we
> _really_ to allocate an skbuf is when we need to put the packet onto a
> queue. All the other use cases are really just to pass a structure
> containing a packet from function to function. For that purpose we
> should be able to just pass a much smaller structure in a stack
> argument and only allocate an skbuff when we need to enqueue. In cases
> where we don't ever queue a packet we might never need to allocate any
> skbuff
Now this intrigues me, because one of the objections to bundling (vs GRO)
was the memory usage of all those SKBs.  IIRC we already do a 'GRO-like'
coalescing when packets reach a TCP socket anyway (or at least in some
cases, not sure if all the different ways we can enqueue a TCP packet for
RX do it), but if we could carry the packets from NIC to socket without
SKBs, doing so in lists rather than one-at-a-time wouldn't cost any extra
memory (the packet-pages are all already allocated on the NIC RX ring).
Possibly combine the two, so that rather than having potentially four
versions of each function (skb, skbundle, void*, void* bundle) you just
have the two 'ends'.

-Ed