netdev - Re: [PATCH 1/2] Move hash calculation inside udp4_lib

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Date: Wed, 26 Jul 2023 16:03:51 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: David Laight <David.Laight@...lab.com>
Cc: Paolo Abeni <pabeni@...hat.com>, 
	"willemdebruijn.kernel@...il.com" <willemdebruijn.kernel@...il.com>, 
	"davem@...emloft.net" <davem@...emloft.net>, "dsahern@...nel.org" <dsahern@...nel.org>, 
	"kuba@...nel.org" <kuba@...nel.org>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/2] Move hash calculation inside udp4_lib_lookup2()

On Wed, Jul 26, 2023 at 4:02 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Paolo Abeni
> > Sent: 26 July 2023 14:44
> >
> > On Wed, 2023-07-26 at 12:05 +0000, David Laight wrote:
> > > Pass the udptable address into udp4_lib_lookup2() instead of the hash slot.
> > >
> > > While ipv4_portaddr_hash(net, IP_ADDR_ANY, 0) is constant for each net
> > > (the port is an xor) the value isn't saved.
> > > Since the hash function doesn't get simplified when passed zero the hash
> >
> > Are you sure? could you please objdump and compare the binary code
> > generated before and after the patch? In theory all the callers up to
> > __jhash_final() included should be inlined, and the compiler should be
> > able to optimze at least rol32(0, <n>).
>
> I looked the hash is 20+ instructions and pretty much all of
> them appeared twice.
> (I'm in the wrong building to have a buildable kernel tree.)
>
> It has to be said that ipv4_portaddr_hash(net, IPADDR_ANY, port)
> could just be net_hash_mix(net) ^ port.
> (Or maybe you could use a different random value.)
>
> I'm not even sure the relatively expensive mixing of 'saddr'
> is needed - it is one of the local IPv4 addresses.
> Mixing in the remote address for connected sockets might
> be useful for any server code that uses a lot of connected
> udp sockets - but that isn't done.
>
> We will have hundreds of udp sockets with different ports that
> are not connected (I don't know if they get bound to a local
> address). So a remote address hash wouldn't help.
>
> If you look at the generated code for __udp4_lib_lookup()
> it is actually quite horrid.
> Too many called functions with too many parameters.
> Things spill to the stack all the time.
>
> The reuse_port code made it a whole lot worse.
>

Maybe ... Please show us performance numbers.

If less than 1%, I would not bother changing this code, making future
backports more risky.