netdev - Re: [PATCH net-next 3/6] flow_dissector: Add hash_extra field to flow

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Message-ID: <20150301194348.GB23622@breakpoint.cc>
Date:	Sun, 1 Mar 2015 20:43:48 +0100
From:	Florian Westphal <fw@...len.de>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Florian Westphal <fw@...len.de>,
	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 3/6] flow_dissector: Add hash_extra field to
 flow_keys struct

Tom Herbert <therbert@...gle.com> wrote:
> We still need hash perturbation for the mapping to a small number of
> queues which can be done after retrieving skb_get_hash, but the
> probability that two different flows match perfectly in skb_get_hash()
> should be 1/2^32-- so are hash collisions really a concern here?

I'm not concerned about accidental collisions, how predictable is skb_get_hash()?
Is skb_get_hash() guaranteed to e.g. contain L4 information?

AFAIK answer to both is "depends on nic/driver", so I feel its better to use
software flow dissector.
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