linux-kernel - Re: [RFC net-next 1/9] skb: introduce gro

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Date: Fri, 21 Jun 2024 09:29:49 -0500
From: Yan Zhai <yan@...udflare.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, 
	Willem de Bruijn <willemb@...gle.com>, Simon Horman <horms@...nel.org>, Florian Westphal <fw@...len.de>, 
	Mina Almasry <almasrymina@...gle.com>, Abhishek Chauhan <quic_abchauha@...cinc.com>, 
	David Howells <dhowells@...hat.com>, Alexander Lobakin <aleksander.lobakin@...el.com>, 
	David Ahern <dsahern@...nel.org>, Richard Gobert <richardbgobert@...il.com>, 
	Antoine Tenart <atenart@...nel.org>, Felix Fietkau <nbd@....name>, 
	Soheil Hassas Yeganeh <soheil@...gle.com>, Pavel Begunkov <asml.silence@...il.com>, 
	Lorenzo Bianconi <lorenzo@...nel.org>, Thomas Weißschuh <linux@...ssschuh.net>, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [RFC net-next 1/9] skb: introduce gro_disabled bit

On Fri, Jun 21, 2024 at 4:49 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Thu, 2024-06-20 at 15:19 -0700, 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@...udflare.com>
> > ---
> >  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
>
> This will generate OoO if the gro_disabled is flipped in the middle of
> a stream.
>
> Assuming the above is fine for your use case (I think it's _not_ in
> general), you could get the same result without an additional costly
> bit in sk_buff.

Calling it per-packet control seems inaccurate here, the motivation is
to give users the ability to control per-flow behaviors. OoO is indeed
a consequence if users don't do it correctly.

>
> Let xdp_frame_fixup_skb_offloading() return a bool - e.g. 'true' when
> gro should be avoided - and let the NIC driver call netif_receive_skb()
> instead of the gro rx hook for such packet.
>
For rx on a single device, directly calling netif_receive_skb is
reasonable. For tunnel receivers it is kinda inconsistent IMHO. For
example, we terminate GRE tunnels in a netns, and it is necessary to
disable GRO on both the entering veth device and also the GRE tunnel
to shutdown GRO. That's why I'd hope to use a bit of skb, to be
consistent within the same netns. Let me add a bit more context to
clarify why we think this is necessary in another thread.

best,
Yan

> All in all the approach implemented in this series does not look worthy
> to me.
>
> Thanks,
>
> Paolo
>