From: Jakub Sitnicki
> Sent: 20 August 2020 11:30
> Subject: Re: BPF sk_lookup v5 - TCP SYN and UDP 0-len flood benchmarks
>
> On Tue, Aug 18, 2020 at 08:19 PM CEST, 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).
>
> Thanks for the tip. I'll give it a try.
You probably don't want to use 'rep stos' in any of its forms.
The instruction 'setup' time is horrid on most cpu variants.
For a 48 byte structure six writes of a zero register will be faster.
If gcc is generating the 'rep stos' then the compiler source code for that
pessimisation needs deleting...
David
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