Re: [RFC net-next 1/9] skb: introduce gro_disabled bit

On Fri, Jun 21, 2024 at 8:13 AM Daniel Borkmann [off-list ref] wrote:

On 6/21/24 2:15 PM, Willem de Bruijn wrote:

quoted

Yan Zhai wrote:

quoted

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 <redacted>
---
  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;
+#endif

Yet 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.

Metadata is in fact one of our interests as well. We discussed using
metadata instead of a skb bit to carry this information internally.
Since metadata is opaque atm so it seems the only option is to have a
GRO control hook before napi_gro_receive, and let BPF decide
netif_receive_skb or napi_gro_receive (echo what Paolo said). With BPF
it could indeed be more flexible, but the cons is that it could be
even more slower than taking a bit on skb. I am actually open to
either approach, as long as it gives us more control on when to enable
GRO :)

To extend the discussion a bit, putting GRO aside, I think some common
hook before GRO would be still valuable moving forward: it is a
limited window where the driver code has both access to XDP context
and skb. Today we do not have a good way to transfer HW offloading
info to skbs if XDP redirect-to-cpu or if XDP encap-and-tx for load
balancing purposes. The XDP metadata infrastructure already allows XDP
to read this information with driver supports, so to complete that, a
place to use it (which I introduced as
xdp_buff/frame_fixup_skb_offloading in a later patch) would be
beneficial to pass on things like the flow hash, vlan information,
etc.

best
Yan