add udp-dont-frag option to support force setting DF bit

[dust-li]

When measuring maximum packets-per-second (PPS) on Linux using multiple sockperf processes to stress UDP throughput, the kernel’s per-packet atomic operation for generating IP ID becomes a bottleneck.

Setting the Don’t Fragment (DF) bit in IP headers allows the kernel to skip this atomic IP ID assignment (as DF packets can use a constant or zero ID), reducing contention and improving scalability under high load.

• Testing:

for((i=0;i<64;i++)); do
    taskset -c $i sockperf tp -i 200.16.10.2 -t 100 --udp-dont-frag
done

• perf top without udp-dont-frag
  56.56% [kernel] [k] ip_idents_reserve
  5.98% [kernel] [k] __dev_xmit_skb
  3.73% [kernel] [k] skb_set_owner_w
  2.37% [kernel] [k] mlx5e_xmit
  2.08% [kernel] [k] mlx5e_consume_skb
  1.63% [kernel] [k] mlx5e_poll_tx_cq
  1.43% [kernel] [k] mlx5e_handle_tx_dim
  1.28% [kernel] [k] sock_wfree
  1.12% [kernel] [k] __slab_free
  0.64% [kernel] [k] build_detached_freelist
  0.61% [kernel] [k] fib_table_lookup
  0.59% sockperf [.] client_handler<IoRecvfrom, SwitchOff, PongModeNever>
  0.57% [kernel] [k] pfifo_fast_dequeue
  0.51% [kernel] [k] udp_sendmsg
  0.50% libpthread-2.32.so [.] __libc_sendto
  0.50% [kernel] [k] __ip_append_data

• Without this option:
  05:23:49 PM IFACE rxpck/s txpck/s rxkB/s txkB/s rxcmp/s txcmp/s rxmcst/s %ifutil
  05:23:50 PM lo 22.00 22.00 8.61 8.61 0.00 0.00 0.00 0.00
  05:23:50 PM eth0 2.00 7958649.00 0.20 466327.09 0.00 0.00 1.00 1.91
  05:23:50 PM eth1 1.00 6588679.00 0.12 386055.41 0.00 0.00 1.00 1.58
  05:23:50 PM bond0 3.00 14547313.00 0.32 852381.62 0.00 0.00 2.00 1.75

• With this option:
  Average: IFACE rxpck/s txpck/s rxkB/s txkB/s rxcmp/s txcmp/s rxmcst/s %ifutil
  Average: lo 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
  Average: eth0 1.00 12339901.50 0.10 723041.10 0.00 0.00 0.50 2.96
  Average: eth1 1.00 13850533.00 0.10 811554.67 0.00 0.00 0.50 3.32
  Average: bond0 2.00 26190604.50 0.20 1534605.73 0.00 0.00 1.00 3.14

[svc-nbu-swx-media]

Can one of the admins verify this patch?

[dust-li]

@igor-ivanov @agalanin-at-nvidia @AlexanderGrissik