Re: [PATCH net-next 4/4] net: add heuristic for enabling TCP fraglist GRO

[Felix Fietkau]

On 24.04.24 20:23, Eric Dumazet wrote:
> On Wed, Apr 24, 2024 at 8:05 PM Felix Fietkau <nbd@xxxxxxxx> wrote:
> > When forwarding TCP after GRO, software segmentation is very expensive,
> > especially when the checksum needs to be recalculated.
> > One case where that's currently unavoidable is when routing packets over
> > PPPoE. Performance improves significantly when using fraglist GRO
> > implemented in the same way as for UDP.
> >
> > When NETIF_F_GRO_FRAGLIST is enabled, perform a lookup for an established
> > socket in the same netns as the receiving device. While this may not
> > cover all relevant use cases in multi-netns configurations, it should be
> > good enough for most configurations that need this.
> >
> > Here's a measurement of running 2 TCP streams through a MediaTek MT7622
> > device (2-core Cortex-A53), which runs NAT with flow offload enabled from
> > one ethernet port to PPPoE on another ethernet port + cake qdisc set to
> > 1Gbps.
> >
> > rx-gro-list off: 630 Mbit/s, CPU 35% idle
> > rx-gro-list on:  770 Mbit/s, CPU 40% idle
> >
> > Signe-off-by: Felix Fietkau <nbd@xxxxxxxx>
> > ---
> >  net/ipv4/tcp_offload.c   | 45 ++++++++++++++++++++++++++++++++++++++-
> >  net/ipv6/tcpv6_offload.c | 46 +++++++++++++++++++++++++++++++++++++++-
> >  2 files changed, 89 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
> > index 6294e7a5c099..f987e2d8423a 100644
> > --- a/net/ipv4/tcp_offload.c
> > +++ b/net/ipv4/tcp_offload.c
> > @@ -404,6 +404,49 @@ void tcp_gro_complete(struct sk_buff *skb)
> >  }
> >  EXPORT_SYMBOL(tcp_gro_complete);
> >
> > +static bool tcp4_check_fraglist_gro(struct sk_buff *skb)
> > +{
> > +       const struct iphdr *iph = skb_gro_network_header(skb);
> > +       struct net *net = dev_net(skb->dev);
> > +       unsigned int off, hlen, thlen;
> > +       struct tcphdr *th;
> > +       struct sock *sk;
> > +       int iif, sdif;
> > +
> > +       if (!(skb->dev->features & NETIF_F_GRO_FRAGLIST))
> > +               return false;
> > +
> > +       inet_get_iif_sdif(skb, &iif, &sdif);
> > +
> > +       off = skb_gro_offset(skb);
> > +       hlen = off + sizeof(*th);
> > +       th = skb_gro_header(skb, hlen, off);
> > +       if (unlikely(!th))
> > +               return false;
> > +
> > +       thlen = th->doff * 4;
> > +       if (thlen < sizeof(*th))
> > +               return false;
> > +
> > +       hlen = off + thlen;
> > +       if (!skb_gro_may_pull(skb, hlen)) {
> > +               th = skb_gro_header_slow(skb, hlen, off);
> > +               if (unlikely(!th))
> > +                       return false;
> > +       }
> > +
> > +       sk = __inet_lookup_established(net, net->ipv4.tcp_death_row.hashinfo,
> > +                                      iph->saddr, th->source,
> > +                                      iph->daddr, ntohs(th->dest),
> > +                                      iif, sdif);
>
> Presumably all this could be done only for the first skb/segment of a GRO train.
>
> We could store the fraglist in a single bit in NAPI_GRO_CB(skb) ?
>
> GRO does a full tuple evaluation, we can trust it.

I will look into that, thanks.

- Felix