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

[Haiyang Zhang]

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
> Sent: Thursday, January 7, 2016 8:28 AM
> To: Simon Xiao <sixiao@xxxxxxxxxxxxx>; Eric Dumazet
> <eric.dumazet@xxxxxxxxx>
> Cc: Tom Herbert <tom@xxxxxxxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx; KY
> Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>;
> devel@xxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; David Miller
> <davem@xxxxxxxxxxxxx>
> Subject: Re: [PATCH net-next] hv_netvsc: don't make assumptions on
> struct flow_keys layout
> 
> Eric Dumazet <eric.dumazet@xxxxxxxxx> writes:
> 
> > On Thu, 2016-01-07 at 10:33 +0100, Vitaly Kuznetsov wrote:
> >> 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.

The Hyper-V vRSS protocol lets us use the Toeplitz hash algorithm. We are
currently running further tests, including IPv6 too, and will share the 
results when available.

Thanks,
- Haiyang