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

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Date:	Thu, 07 Jan 2016 14:28:26 +0100
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	Simon Xiao <sixiao@...rosoft.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tom Herbert <tom@...bertland.com>, netdev@...r.kernel.org,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next] hv_netvsc: don't make assumptions on struct flow_keys layout

Eric Dumazet <eric.dumazet@...il.com> 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.


> 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


View attachment "0001-hv_netvsc-use-skb_get_hash-instead-of-a-homegrown-im.patch" of type "text/x-patch" (2421 bytes)