netdev - Re: [PATCH net-next 0/6] net: Call skb_get

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Date:	Wed, 04 Mar 2015 12:03:03 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, fw@...len.de
Subject: Re: [PATCH net-next 0/6] net: Call skb_get_hash in qdiscs

On Wed, 2015-03-04 at 10:39 -0800, Tom Herbert wrote:

> - The flow keys are passed in cb[] structure. This severely limits
>   our ability to increase the key space. For instance, the folding
>   of IPv6 addresses in flow_dissector to make a 32-bit value
>   represents a significant loss of information (it would be easy
>   to create millions of different 4-tuples that would always produce the
>   same hash value). 


Yes, but then your patch is all about reducing flow compares to a single
u32 comparison in qdiscs (and elsewhere)

choke for example explicitly wants to make sure we drop a companion
if incoming packet belongs to the same flow.

Relying on a 'strong hash' whatever it can be was not considered in
Choke paper. There is no mention of a stochastic match.

If we no longer can store the keys in skb->cb[], fine (although I claim
skb->cb[] size should be irrelevant, see our discussion on this topic
with Florian)
 -> Just recompute the keys, using a local variable, from packet
content. Yes, it will be more expensive, but hey, we get what we want.

Same for sfq : your skb_get_hash_perturb() doesn't address the point I
made earlier.

It is only giving a false sense of security.
I would rather not spread it.
(Note that there is no documentation or changelog to explain the
pro/cons)

I doubt OVS would condense their flow keys in a single u32...


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