Re: BPF sk_lookup v5 - TCP SYN and UDP 0-len flood benchmarks

On Tue, 2020-08-18 at 11:19 -0700, Alexei Starovoitov wrote:

On Tue, Aug 18, 2020 at 8:49 AM Jakub Sitnicki [off-list ref] wrote:

quoted

         :                      rcu_read_lock();
         :                      run_array = rcu_dereference(net->bpf.run_array[NETNS_BPF_SK_LOOKUP]);
    0.01 :   ffffffff817f8624:       mov    0xd68(%r12),%rsi
         :                      if (run_array) {
    0.00 :   ffffffff817f862c:       test   %rsi,%rsi
    0.00 :   ffffffff817f862f:       je     ffffffff817f87a9 <__udp4_lib_lookup+0x2c9>
         :                      struct bpf_sk_lookup_kern ctx = {
    1.05 :   ffffffff817f8635:       xor    %eax,%eax
    0.00 :   ffffffff817f8637:       mov    $0x6,%ecx
    0.01 :   ffffffff817f863c:       movl   $0x110002,0x40(%rsp)
    0.00 :   ffffffff817f8644:       lea    0x48(%rsp),%rdi
   18.76 :   ffffffff817f8649:       rep stos %rax,%es:(%rdi)
    1.12 :   ffffffff817f864c:       mov    0xc(%rsp),%eax
    0.00 :   ffffffff817f8650:       mov    %ebp,0x48(%rsp)
    0.00 :   ffffffff817f8654:       mov    %eax,0x44(%rsp)
    0.00 :   ffffffff817f8658:       movzwl 0x10(%rsp),%eax
    1.21 :   ffffffff817f865d:       mov    %ax,0x60(%rsp)
    0.00 :   ffffffff817f8662:       movzwl 0x20(%rsp),%eax
    0.00 :   ffffffff817f8667:       mov    %ax,0x62(%rsp)
         :                      .sport          = sport,
         :                      .dport          = dport,
         :                      };

Such heavy hit to zero init 56-byte structure is surprising.
There are two 4-byte holes in this struct. You can try to pack it and
make sure that 'rep stoq' is used instead of 'rep stos' (8 byte at a time vs 4).

I think here rep stos is copying 8 bytes at a time (%rax operand, %ecx
initalized with '6').

I think that you can avoid the costly instruction explicitly
initializing each field individually:

	struct bpf_sk_lookup_kern ctx;

	ctx.family = AF_INET;
	ctx.protocol = protocol;
	// ...

note, you likely want to explicitly zero the v6 addresses, too.

Cheers,

Paolo