netdev - Re: unexpected GRO/veth behavior

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Date:   Tue, 11 Sep 2018 09:33:28 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Re: unexpected GRO/veth behavior

On 2018/09/10 23:56, Eric Dumazet wrote:
> On 09/10/2018 07:44 AM, Paolo Abeni wrote:
>> hi all,
>>
>> while testing some local patches I observed that the TCP tput in the
>> following scenario:
>>
>> # the following enable napi on veth0, so that we can trigger the
>> # GRO path with namespaces
>> ip netns add test
>> ip link add type veth
>> ip link set dev veth0 netns test
>> ip -n test link set lo up
>> ip -n test link set veth0 up
>> ip -n test addr add dev veth0 172.16.1.2/24
>> ip link set dev veth1 up
>> ip addr add dev veth1 172.16.1.1/24
>> IDX=`ip netns exec test cat /sys/class/net/veth0/ifindex`
>>
>> # 'xdp_pass' is a NO-OP XDP program that simply return XDP_PASS
>> ip netns exec test ./xdp_pass $IDX &
>> taskset 0x2 ip netns exec test iperf3 -s -i 60 &
>> taskset 0x1 iperf3 -c 172.16.1.2 -t 60 -i 60
>>
>> is quite lower than expected (~800Mbps). 'perf' shows a weird topmost 
>> offender:
>>
> 
> 
> But... why GRO would even be needed in this scenario ?
> 
> GRO is really meant for physical devices, having to mess with skb->sk adds extra cost
> in this already heavy cost engine.
> 
> Virtual devices should already be fed with TSO packets.

Because XDP does not have SG feature (GRO path in veth is used only when
XDP is enabled).

I have tested configuration like this:

NIC ---(XDP_REDIRECT)---> veth===veth (XDP_PASS)

GRO seems to work and improves TCP throughput in this case.


Now I noticed I did not test:

netperf -> veth===veth (XDP_PASS) -> netserver

which I think is the case where Paolo faces a problem.

I think it is not the case XDP can improve performance. I think I can
disable GRO for packets with skb->sk != NULL in veth.

-- 
Toshiaki Makita