On Tue, 2020-08-18 at 11:19 -0700, Alexei Starovoitov wrote:
> On Tue, Aug 18, 2020 at 8:49 AM Jakub Sitnicki <jakub@xxxxxxxxxxxxxx> wrote:
> > : 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
