Re: [PATCH net-next] hv_netvsc: don't make assumptions on struct flow_keys layout

Eric Dumazet [off-list ref] writes:

On Thu, 2016-01-07 at 10:33 +0100, Vitaly Kuznetsov wrote:

quoted

Recent changes to 'struct flow_keys' (e.g commit d34af823ff40 ("net: Add
 VLAN ID to flow_keys")) introduced a performance regression in netvsc
driver. Is problem is, however, not the above mentioned commit but the
fact that netvsc_set_hash() function did some assumptions on the struct
flow_keys data layout and this is wrong. We need to extract the data we
need (src/dst addresses and ports) after the dissect.

The issue could also be solved in a completely different way: as suggested
by Eric instead of our own homegrown netvsc_set_hash() we could use
skb_get_hash() which does more or less the same. Unfortunately, the
testing done by Simon showed that Hyper-V hosts are not happy with our
Jenkins hash, selecting the output queue with the current algorithm based
on Toeplitz hash works significantly better.

Were tests done on IPv6 traffic ?

Simon, could you please test this patch for IPv6 and show us the numbers?

Toeplitz hash takes at least 100 ns to hash 12 bytes (one iteration per
bit : 96 iterations)

For IPv6 it is 3 times this, since we have to hash 36 bytes.

I do not see how it can compete with skb_get_hash() that directly gives
skb->hash for local TCP flows.

My guess is that this is not the bottleneck, something is happening
behind the scene with out packets in Hyper-V host (e.g. re-distributing
them to hardware queues?) but I don't know the internals, Microsoft
folks could probably comment.

See commits b73c3d0e4f0e1961e15bec18720e48aabebe2109
("net: Save TX flow hash in sock and set in skbuf on xmit")
and 877d1f6291f8e391237e324be58479a3e3a7407c
("net: Set sk_txhash from a random number")

I understand Microsoft loves Toeplitz, but this looks not well placed
here.

I suspect there is another problem.

Please share your numbers and test methodology, and the alternative
patch Simon tested so that we can double check it.

Alternative patch which uses skb_get_hash() attached. Simon, could you
please share the rest (environment, metodology, numbers) with us here?
Thanks!

Thanks.

PS: For the time being this patch can probably be applied on -net tree,
as it fixes a real bug.

-- 
  Vitaly