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Date: Mon, 24 Aug 2020 10:17:03 +0200 From: Paolo Abeni <pabeni@...hat.com> To: Alexei Starovoitov <alexei.starovoitov@...il.com>, Jakub Sitnicki <jakub@...udflare.com> Cc: bpf <bpf@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, kernel-team <kernel-team@...udflare.com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Andrii Nakryiko <andriin@...com>, Lorenz Bauer <lmb@...udflare.com>, Marek Majkowski <marek@...udflare.com>, Martin KaFai Lau <kafai@...com>, Yonghong Song <yhs@...com> Subject: 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 <jakub@...udflare.com> 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
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