On some platforms, GRO stack is too deep and causes cpu stalls.

Decreasing call depths by one shows a 1.5 % gain on Zen2 cpus.
(32 RX queues, 100Gbit NIC, RFS enabled, tcp_rr with 128 threads and 10,000 flows)

We can go further by inlining ipv6_gro_{receive,complete}
and take care of IPv4 if there is interest.

Note: two temporary __always_inline will be replaced with
      inline_for_performance when/if available.

v3: removed INDIRECT_CALLABLE_SCOPE on udp6_gro_receive()
    and udp6_gro_complete(), as gcc complained (Jakub)

v2: dealt with udp6_gro_receive()/udp6_gro_complete()
    missing declarations (kernel test robot [off-list ref])
    for CONFIG_MITIGATION_RETPOLINE=n

Cumulative size increase for this series (of 3):

$ scripts/bloat-o-meter -t vmlinux.0 vmlinux.3
add/remove: 2/2 grow/shrink: 5/1 up/down: 1572/-471 (1101)
Function                                     old     new   delta
ipv6_gro_receive                            1069    1846    +777
ipv6_gro_complete                            433     733    +300
tcp6_check_fraglist_gro                        -     272    +272
tcp6_gro_complete                            227     306     +79
tcp4_gro_complete                            325     397     +72
ipv6_offload_init                            218     274     +56
__pfx_tcp6_check_fraglist_gro                  -      16     +16
__pfx___skb_incr_checksum_unnecessary         32       -     -32
__skb_incr_checksum_unnecessary              186       -    -186
tcp6_gro_receive                             959     706    -253
Total: Before=22592724, After=22593825, chg +0.00%

Eric Dumazet (3):
  net: always inline __skb_incr_checksum_unnecessary()
  gro: inline tcp6_gro_receive()
  gro: inline tcp6_gro_complete()

 include/linux/skbuff.h   |  2 +-
 include/net/gro.h        |  5 ++---
 include/net/tcp.h        |  2 --
 net/ipv6/Makefile        |  2 +-
 net/ipv6/ip6_offload.c   | 43 ++++++++++++++++++++--------------------
 net/ipv6/tcpv6_offload.c | 12 +++++------
 net/ipv6/udp_offload.c   |  3 +--
 7 files changed, 32 insertions(+), 37 deletions(-)

-- 
2.52.0.457.g6b5491de43-goog