Re: [PATCH net-next 1/3] net: gro: set {inner_,}network_header in receive phase

Richard Gobert
2024-02-29T13:22:36Z

Eric Dumazet wrote:
>
> My intuition is that this patch has a high cost for normal GRO processing.
> SW-GRO is already a bottleneck on ARM cores in smart NICS.
>
> I would suggest instead using parameters to give both the nhoff and thoff values
> this would avoid many conditionals in the fast path.
>
> ->
>
> INDIRECT_CALLABLE_SCOPE int udp6_gro_complete(struct sk_buff *skb, int
> nhoff, int thoff)
> {
> const struct ipv6hdr *ipv6h = (const struct ipv6hdr *)(skb->data + nhoff);
> struct udphdr *uh = (struct udphdr *)(skb->data + thoff);
> ...
> }
>
> INDIRECT_CALLABLE_SCOPE int tcp6_gro_complete(struct sk_buff *skb, int
> nhoff, int thoff)
> {
> const struct ipv6hdr *iph = (const struct ipv6hdr *)(skb->data + nhoff);
> struct tcphdr *th = (struct tcphdr *)(skb->data + thoff);
>
> Why storing in skb fields things that really could be propagated more
> efficiently as function parameters ?

Hi Eric,
Thanks for the review!

I agree, the conditionals could be a problem and are actually not needed.
The third commit in this patch series introduces an optimisation for
ipv6/ipv4 using the correct {inner_}network_header. We can remove the
conditionals; I thought about multiple ways to do so. First, remove the
conditional in skb_gro_network_offset:

static inline int skb_gro_network_offset(const struct sk_buff *skb)
{
const u32 mask = NAPI_GRO_CB(skb)->encap_mark - 1;
return (skb_network_offset(skb) & mask) | (skb_inner_network_offset(skb) & ~mask);
}

And for the conditionals in {inet,ipv6}_gro_receive I thought about two
ideas. The first is to move set_inner_network_header to encapsulation gro
functions like ipip_gro_receive, this way there's one less write (in
comparison to main) in these functions:

static struct sk_buff *ipip_gro_receive(struct list_head *head,
struct sk_buff *skb)
{
...

NAPI_GRO_CB(skb)->encap_mark = 1;
skb_set_inner_network_header(skb, skb_gro_offset(skb));

The second way is to always write to inner_network_header:

INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
struct sk_buff *skb)
{
...
skb_set_inner_network_header(skb, off);
...

What do you think is better? I think the 1st is more beneficial for the
fast path.

We could then use the {inner_}network_header separation to optimise the
receive path, such as in the 3rd commit in this patch series.

Regards,
Richard