From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH net-next v4] rps: selective flow shedding during softnet
 overflow
Date: Wed, 24 Apr 2013 07:41:04 -0700
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Cc: Stephen Hemminger <stephen@networkplumber.org>,
	Willem de Bruijn <willemb@google.com>,
	netdev@vger.kernel.org, davem@davemloft.net
To: Jamal Hadi Salim <jhs@mojatatu.com>
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On Wed, 2013-04-24 at 09:00 -0400, Jamal Hadi Salim wrote:
> I forgot about the Toepliz hash connection. I can see it makes sense here.
> 
> Let me clarify:
> In the scenario i am thinking of, I have clever hardware which is smart 
> enough to deal with details of identifying flow state(including 
> fragementation etc) and tagging it in a DMA descriptor with 32 bit id.
> I want to be able to take the tag produced by the hardware and use
> that for rps cpu selection i.e assume the hardware has already done the
> hashing and is giving me a 32 bit id. My initial thought was skb->rxhash
> is the right spot to store this; then make get_rps_cpu() do the
> selection based on this. l4 rxhash is 1 bit which is too small.

Set skb->rxrhash to the hash your hardware computed, and skb->l4_rxhash
to 1.

Then get_rps_cpu() will use skb->rxhash happily

(and other callers of skb_get_rxhash() as well)

Not clear what you mean by fragmentation : fragmented frames have no
flow information (but the first fragment)