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

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Date: Wed, 26 Jul 2023 14:02:33 +0000
From: David Laight <David.Laight@...LAB.COM>
To: '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>, "'Eric
 Dumazet'" <edumazet@...gle.com>, "'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()

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.

	David

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