netdev - Re: Get rxhash fixes and RFS support in tun

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Date:	Thu, 21 Nov 2013 08:28:18 -0800
From:	Tom Herbert <therbert@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	Linux Netdev List <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Jerry Chu <hkchu@...gle.com>
Subject: Re: Get rxhash fixes and RFS support in tun

On Wed, Nov 20, 2013 at 6:50 PM, David Miller <davem@...emloft.net> wrote:
> From: Tom Herbert <therbert@...gle.com>
> Date: Wed, 20 Nov 2013 18:42:48 -0800
>
>> We need the rxhash to be the value seen at the point of RPS (to do RFS
>> correctly), which I think probably means we don't ever want to change
>> it after the first calculation! (clearing at tunnel decap wouldn't be
>> correct either)  For ESP or AH, I believe it's appropriate to use SPI
>> as a substitute for ports.
>
> But that means that all connections going over the same IPSEC path
> hash to the same value.
>
> I really think that xfrm_input() should zap the rxhash near the
> existing nf_reset() call.
>
> The same argument goes for tunnels, that is why ip_tunnel_core.c does
> what it does with the rxhash clearing right now.
>
I suspect this is not the right thing to do any more.  Since we're
doing deep inspection now in flow_dissector, we should already have
the discovered the hash on the inner header if it's a standard encap.
In order to do RFS for tunneled packets at the NIC interface we don't
want recompute to a different L4 hash (which would be common case if
NIC did deep inspection on tunneled packets and we clear hash at
decap).

> In both the IPSEC tunnel (not transport) and normal IP/GRE tunnel
> cases, it's a completely new SKB receive, done via netif_rx(), after
> decapsulation.
>
> That leaves only IPSEC transport mode as the only case where RFS isn't
> (re-)performed but we can build infrastructure to make that happen.
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