Re: [RFC PATCH tip 0/5] tracing filters with BPF

[Ingo Molnar <mingo@kernel.org>]

* Alexei Starovoitov <ast@plumgrid.com> wrote:

> > On Tue, Dec 3, 2013 at 4:01 PM, Andi Kleen <andi@firstfloor.org> wrote:
> >>
> >> Can you do some performance comparison compared to e.g. ktap?
> >> How much faster is it?
> 
> Did simple ktap test with 1M alloc_skb/kfree_skb toy test from earlier email:
> trace skb:kfree_skb {
>         if (arg2 == 0x100) {
>                 printf("%x %x\n", arg1, arg2)
>         }
> }
> 1M skb alloc/free 350315 (usecs)
> 
> baseline without any tracing:
> 1M skb alloc/free 145400 (usecs)
> 
> then equivalent bpf test:
> void filter(struct bpf_context *ctx)
> {
>         void *loc = (void *)ctx->regs.dx;
>         if (loc == 0x100) {
>                 struct sk_buff *skb = (struct sk_buff *)ctx->regs.si;
>                 char fmt[] = "skb %p loc %p\n";
>                 bpf_trace_printk(fmt, sizeof(fmt), (long)skb, (long)loc, 0);
>         }
> }
> 1M skb alloc/free 183214 (usecs)
> 
> so with one 'if' condition the difference ktap vs bpf is 350-145 vs 183-145
> 
> obviously ktap is an interpreter, so it's not really fair.
> 
> To make it really unfair I did:
> trace skb:kfree_skb {
>         if (arg2 == 0x100 || arg2 == 0x200 || arg2 == 0x300 || arg2 == 0x400 ||
>             arg2 == 0x500 || arg2 == 0x600 || arg2 == 0x700 || arg2 == 0x800 ||
>             arg2 == 0x900 || arg2 == 0x1000) {
>                 printf("%x %x\n", arg1, arg2)
>         }
> }
> 1M skb alloc/free 484280 (usecs)

Real life scripts, for examples the ones related to network protocol 
analysis will often have such patterns in them, so I don't think this 
measurement is particularly unfair.

Thanks,

	Ingo