Re: [PATCH v4 net-next v4 6/6] net: add heuristic for enabling TCP fraglist GRO

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2024-04-30 at 12:23 +0200, Felix Fietkau wrote:
> On 30.04.24 12:12, Paolo Abeni wrote:
> > On Sat, 2024-04-27 at 20:23 +0200, Felix Fietkau 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@nbd.name>
> > > ---
> > >  net/ipv4/tcp_offload.c   | 32 ++++++++++++++++++++++++++++++++
> > >  net/ipv6/tcpv6_offload.c | 35 +++++++++++++++++++++++++++++++++++
> > >  2 files changed, 67 insertions(+)
> > > 
> > > diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
> > > index 87ae9808e260..3e9b8c6f9c8c 100644
> > > --- a/net/ipv4/tcp_offload.c
> > > +++ b/net/ipv4/tcp_offload.c
> > > @@ -407,6 +407,36 @@ void tcp_gro_complete(struct sk_buff *skb)
> > >  }
> > >  EXPORT_SYMBOL(tcp_gro_complete);
> > >  
> > > +static void tcp4_check_fraglist_gro(struct list_head *head, struct sk_buff *skb,
> > > +				    struct tcphdr *th)
> > > +{
> > > +	const struct iphdr *iph;
> > > +	struct sk_buff *p;
> > > +	struct sock *sk;
> > > +	struct net *net;
> > > +	int iif, sdif;
> > > +
> > > +	if (!(skb->dev->features & NETIF_F_GRO_FRAGLIST))
> > 
> > Should we add an 'unlikely()' here to pair with unlikely(is_flist) in
> > *gro_receive / *gro_complete?
> Not sure if unlikely() will make any difference here. I think it makes 
> more sense in the other places than here.

Why? AFAICS this will be called for every packet on the wire, exactly
as the code getting this annotation in patch 3/6.

> > Should this test be moved into the caller, to avoid an unconditional
> > function call in the ipv6 code?
> 
> The function is already called from tcp4_gro_receive, which is only 
> called by IPv4 code. Also, since it's a static function called in only 
> one place, it gets inlined by the compiler (at least in my builds).
> Not sure what unconditional function call you're referring to.

Right you are. I just got fooled by my hope to reuse the same function
for ipv4 and v6. Please just ignore this last part.

Cheers,

Paolo