On 6/21/24 2:15 PM, Willem de Bruijn wrote:
Yan Zhai wrote:Software GRO is currently controlled by a single switch, i.e. ethtool -K dev gro on|off However, this is not always desired. When GRO is enabled, even if the kernel cannot GRO certain traffic, it has to run through the GRO receive handlers with no benefit. There are also scenarios that turning off GRO is a requirement. For example, our production environment has a scenario that a TC egress hook may add multiple encapsulation headers to forwarded skbs for load balancing and isolation purpose. The encapsulation is implemented via BPF. But the problem arises then: there is no way to properly offload a double-encapsulated packet, since skb only has network_header and inner_network_header to track one layer of encapsulation, but not two. On the other hand, not all the traffic through this device needs double encapsulation. But we have to turn off GRO completely for any ingress device as a result. Introduce a bit on skb so that GRO engine can be notified to skip GRO on this skb, rather than having to be 0-or-1 for all traffic. Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx> --- include/linux/netdevice.h | 9 +++++++-- include/linux/skbuff.h | 10 ++++++++++ net/Kconfig | 10 ++++++++++ net/core/gro.c | 2 +- net/core/gro_cells.c | 2 +- net/core/skbuff.c | 4 ++++ 6 files changed, 33 insertions(+), 4 deletions(-) diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h index c83b390191d4..2ca0870b1221 100644 --- a/include/linux/netdevice.h +++ b/include/linux/netdevice.h @@ -2415,11 +2415,16 @@ struct net_device { ((dev)->devlink_port = (port)); \ })-static inline bool netif_elide_gro(const struct net_device *dev)+static inline bool netif_elide_gro(const struct sk_buff *skb) { - if (!(dev->features & NETIF_F_GRO) || dev->xdp_prog) + if (!(skb->dev->features & NETIF_F_GRO) || skb->dev->xdp_prog) return true; + +#ifdef CONFIG_SKB_GRO_CONTROL + return skb->gro_disabled; +#else return false; +#endifYet more branches in the hot path. Compile time configurability does not help, as that will be enabled by distros. For a fairly niche use case. Where functionality of GRO already works. So just a performance for a very rare case at the cost of a regression in the common case. A small regression perhaps, but death by a thousand cuts.
Mentioning it here b/c it perhaps fits in this context, longer time ago there was the idea mentioned to have BPF operating as GRO engine which might also help to reduce attack surface by only having to handle packets of interest for the concrete production use case. Perhaps here meta data buffer could be used to pass a notification from XDP to exit early w/o aggregation.
