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

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Date:	Fri, 6 Dec 2013 14:17:15 +0800
From:	Jovi Zhangwei <jovi.zhangwei@...il.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Tom Zanussi <tom.zanussi@...ux.intel.com>,
	Eric Dumazet <edumazet@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH tip 0/5] tracing filters with BPF

On Thu, Dec 5, 2013 at 12:40 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
>> On Tue, Dec 3, 2013 at 4:01 PM, Andi Kleen <andi@...stfloor.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)
>
I've lost my mind for a while. :)

If bpf only focus on filter, then it's not good to compare with ktap
like that, since
ktap can easily make use on current kernel filter, you should use below script:

trace skb:kfree_skb /location == 0x100 || location == 0x200 || .../ {
    printf("%x %x\n", arg1, arg2)
}

As ktap is a user of current simple kernel tracing filter, I fully
agree with Steven,
    "it can be an add on, but not a replacement."


Thanks,

Jovi
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