Subject: Re: Performance regression 3.11 with macvlan between 2 linux guests (bisected)

From: Vlad Yasevich <[email-masked]>
Date: Wed, 21 Aug 2013 14:04:53 -0400

On 08/21/2013 08:38 AM, Christian Borntraeger wrote:
> Vlad,
>
> the patch
>
> commit 3e4f8b787370978733ca6cae452720a4f0c296b8
> Author: Vlad Yasevich <[email-masked]>
> Date:   Tue Jun 25 16:04:22 2013 -0400
>
>      macvtap: Perform GSO on forwarding path.
>
> causes a severe performance regression for 2 Linux guests with virtio-net
> connected via macvlan/vtap doing iperf workload. (2GBit vs. 20Gbit)
>
> If I understand the patch correctly, we now check for gso depending on the
> macvlan features. If the underlying hardware does not support the necessary
> offloads then we will do segmentation, even if we keep the whole traffic internal
> between two guests and even if both guests supports LRO,GSO etc.
> ---snip---
> [...]
> +       features = netif_skb_features(skb) & vlan->tap_features;
> +       if (netif_needs_gso(skb, features)) {
> +               struct sk_buff *segs = __skb_gso_segment(skb, features, false);
> [...]
>
> ---snip---
> Shouldnt we take the features of the target device or even better do the gsoing in the
> target device driver?

A corrected patch has been sent upstream.  We take into consideration 
the features on the target device that the user/vm has specified.
If the VM has enabled the TSO flags, then nothing will happen to the
GSO packet.  However, if the TSO flag is off, segmentation will be 
performed.

-vlad

>
> Christian
>
> FYI, the underlying HW has:
>
> Features for eth0:
> rx-checksumming: off [fixed]
> tx-checksumming: off
> 	tx-checksum-ipv4: off [fixed]
> 	tx-checksum-ip-generic: off [fixed]
> 	tx-checksum-ipv6: off [fixed]
> 	tx-checksum-fcoe-crc: off [fixed]
> 	tx-checksum-sctp: off [fixed]
> scatter-gather: off
> 	tx-scatter-gather: off [fixed]
> 	tx-scatter-gather-fraglist: off [fixed]
> tcp-segmentation-offload: off
> 	tx-tcp-segmentation: off [fixed]
> 	tx-tcp-ecn-segmentation: off [fixed]
> 	tx-tcp6-segmentation: off [fixed]
> udp-fragmentation-offload: off [fixed]
> generic-segmentation-offload: off [requested on]
> generic-receive-offload: on
> large-receive-offload: off [fixed]
> rx-vlan-offload: off [fixed]
> tx-vlan-offload: off [fixed]
> ntuple-filters: off [fixed]
> receive-hashing: off [fixed]
> highdma: off [fixed]
> rx-vlan-filter: on [fixed]
> vlan-challenged: off [fixed]
> tx-lockless: off [fixed]
> netns-local: off [fixed]
> tx-gso-robust: off [fixed]
> tx-fcoe-segmentation: off [fixed]
> tx-gre-segmentation: off [fixed]
> tx-udp_tnl-segmentation: off [fixed]
> tx-mpls-segmentation: off [fixed]
> fcoe-mtu: off [fixed]
> tx-nocache-copy: off
> loopback: off [fixed]
> rx-fcs: off [fixed]
> rx-all: off [fixed]
> tx-vlan-stag-hw-insert: off [fixed]
> rx-vlan-stag-hw-parse: off [fixed]
> rx-vlan-stag-filter: off [fixed]
>