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

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Date:	Wed, 4 Mar 2015 13:49:38 -0800
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Florian Westphal <fw@...len.de>
Subject: Re: [PATCH net-next 0/6] net: Call skb_get_hash in qdiscs

On Wed, Mar 4, 2015 at 12:03 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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.
>
Then choke is broke :-) ipv6_addr_hash already makes the "flow" match
stochastic.

> 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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