netdev - Re: Scaling problem with a lot of AF_PACKET sockets on different interfaces

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Message-ID: <51B1EF27.9030300@redhat.com>
Date:	Fri, 07 Jun 2013 16:33:11 +0200
From:	Daniel Borkmann <dborkman@...hat.com>
To:	"Vitaly V. Bursov" <vitalyb@...enet.dn.ua>
CC:	Mike Galbraith <bitbucket@...ine.de>, linux-kernel@...r.kernel.org,
	netdev <netdev@...r.kernel.org>
Subject: Re: Scaling problem with a lot of AF_PACKET sockets on different
 interfaces

On 06/07/2013 04:17 PM, Vitaly V. Bursov wrote:
> 07.06.2013 16:05, Daniel Borkmann пишет:
[...]
>>>> Ideas are welcome :)
>>
>> Probably, that depends on _your scenario_ and/or BPF filter, but would it be
>> an alternative if you have only a few packet sockets (maybe one pinned to each
>> cpu) and cluster/load-balance them together via packet fanout? (Where you
>> bind the socket to ifindex 0, so that you get traffic from all devs...) That
>> would at least avoid that "hot spot", and you could post-process the interface
>> via sockaddr_ll. But I'd agree that this will not solve the actual problem you've
>> observed. ;-)
>
> I was't aware of the ifindex 0 thing, it can help, thanks! Of course, if it'll
> work for me (applications is a custom DHCP server) it'll surely
> increase the overhead of BPF (I don't need to tap the traffic from all
> interfaces), there are vlans, bridges and bonds - likely the server will receive
> same packets multiple times and replies must be sent too...
> but it still should be faster.

Well, as already said, if you use a fanout socket group, then you won't receive the
_exact_ same packet twice. Rather, packets are balanced by different policies among
your packet sockets in that group. What you could do is to have a (e.g.) single BPF
filter (jitted) for all those sockets that'll let needed packets pass and you can then
access the interface they came from via sockaddr_ll, which then is further processed
in your fast path (or dropped depending on the iface). There's also a BPF extension
(BPF_S_ANC_IFINDEX) that lets you load the ifindex of the skb into the BPF accumulator,
so you could also filter early from there for a range of ifindexes (in combination to
bind the sockets to index 0). Probably that could work.
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