netdev - Re: [PATCH v2 net-next 0/3] ipv4: Hash-based multipath routing

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Message-ID: <CALx6S35AUXFKfazU4bbpNL90r7P8i3wSFTB_0YDDRUgKg-3vqw@mail.gmail.com>
Date:	Sat, 29 Aug 2015 13:59:08 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	David Miller <davem@...emloft.net>
Cc:	Peter Christensen <pch@...bogen.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, linux-api@...r.kernel.org,
	Roopa Prabhu <roopa@...ulusnetworks.com>, sfeldma@...il.com,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	Thomas Graf <tgraf@...g.ch>, Jiri Benc <jbenc@...hat.com>
Subject: Re: [PATCH v2 net-next 0/3] ipv4: Hash-based multipath routing

On Sat, Aug 29, 2015 at 1:46 PM, David Miller <davem@...emloft.net> wrote:
> From: Peter Nørlund <pch@...bogen.com>
> Date: Sat, 29 Aug 2015 22:31:15 +0200
>
>> On Sat, 29 Aug 2015 13:14:29 -0700 (PDT)
>> David Miller <davem@...emloft.net> wrote:
>>
>>> From: pch@...bogen.com
>>> Date: Fri, 28 Aug 2015 22:00:47 +0200
>>>
>>> > When the routing cache was removed in 3.6, the IPv4 multipath
>>> > algorithm changed from more or less being destination-based into
>>> > being quasi-random per-packet scheduling. This increases the risk
>>> > of out-of-order packets and makes it impossible to use multipath
>>> > together with anycast services.
>>>
>>> Don't even try to be fancy.
>>>
>>> Simply kill the round-robin stuff off completely, and make hash based
>>> routing the one and only mode, no special configuration stuff
>>> necessary.
>>
>> I like the sound of that! Just to be clear - are you telling me to
>> stick with L3 and skip the L4 part?
>
> For now it seems best to just do L3 and make ipv4 and ipv6 behave the
> same.

This might be simpler if we just go directly to L4 which should be
better load balancing and what most switches are doing anyway. The
hash comes from:

1) If a lookup includes an skb, we just need to call skb_get_hash.
2) If we have a socket and sk->sk_txhash is nonzero then use that.
3) Else compute a hash frome flowi. We don't have the exact functions
for this, but they can be easily derived from __skb_get_hash_flowi4
and __skb_get_hash_flowi6 (i.e. create general get_hash_flowi4 and
get_hash_flowi6 and then call these from skb functions and multipath
lookup).

Tom

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