Re: [RFC PATCH 1/2] net: gro: avoid relaying on skb->transport_header at receive time

[Paolo Abeni <pabeni@redhat.com>]

On 12/5/25 3:37 PM, Eric Dumazet wrote:
> On Fri, Dec 5, 2025 at 6:04 AM Paolo Abeni <pabeni@redhat.com> wrote:
>>
>> Currently {tcp,udp}_gro_receive relay on the gro network stage setting
> 
> rely :)
> 
>> the correct transport header offset for all the skbs held by the GRO
>> engine.
>>
>> Such assumption is not necessary, as the code can instead leverage the
>> offset already available for the currently processed skb. Add a couple
>> of helpers to for readabilty' sake.
>>
>> As skb->transport_header lays on a different cacheline wrt skb->data,
>> this should save a cacheline access for each packet aggregation.
>> Additionally this will make the next patch possible.
>>
>> Note that the compiler (gcc 15.2.1) does inline the tcp_gro_lookup()
>> call in tcp_gro_receive(), so the additional argument is only relevant
>> for the fraglist case.
>>
>> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
>> ---
>>  include/net/gro.h        | 26 ++++++++++++++++++++++++++
>>  include/net/tcp.h        |  3 ++-
>>  net/ipv4/tcp_offload.c   | 15 ++++++++-------
>>  net/ipv4/udp_offload.c   |  4 ++--
>>  net/ipv6/tcpv6_offload.c |  2 +-
>>  5 files changed, 39 insertions(+), 11 deletions(-)
>>
>> diff --git a/include/net/gro.h b/include/net/gro.h
>> index b65f631c521d..fdb9285ab117 100644
>> --- a/include/net/gro.h
>> +++ b/include/net/gro.h
>> @@ -420,6 +420,18 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
>>                                 struct udphdr *uh, struct sock *sk);
>>  int udp_gro_complete(struct sk_buff *skb, int nhoff, udp_lookup_t lookup);
>>
>> +/* Return the skb hdr corresponding to the specified skb2 hdr.
>> + * skb2 is held in the gro engine, i.e. its headers are in the linear part.
>> + */
>> +static inline const void *
>> +skb_gro_header_from(const struct sk_buff *skb, const struct sk_buff *skb2,
>> +                   const void *hdr2)
>> +{
>> +       size_t offset = (unsigned char *)hdr2 - skb2->data;
>> +
>> +       return skb->data + offset;
>> +}
> 
> I would rather switch gro to pass an @offset instead of a header pointer ?
> 
> Rebuilding one header pointer from offset is fast : skb->data + offset
> ( offset : network header, transport header, ...)

I considered such option and opted for the above for a very small
reason: it produces a little more compact (C) code in the caller.

I'll switch to offset in next revisions.
> As a matter of fact, some GRO state variables could be onstack, instead
> of being stored in NAPI_GRO_CB()
Do you mean the network offsets? In any case, I hope we can keep such
work separate from this one?
> This would avoid some stalls because skb->cb[] has been cleared with
> memset() with long words, while GRO is using smaller fields.Whoops, I never considered store forwarding induced stalls. Something to
ponder about for me.

Many thanks!

Paolo