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

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Date:	Wed, 4 Dec 2013 20:40:44 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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>,
	Jovi Zhangwei <jovi.zhangwei@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH tip 0/5] tracing filters with BPF

> 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)

and corresponding bpf:
void filter(struct bpf_context *ctx)
{
        void *loc = (void *)ctx->regs.dx;
        if (loc == 0x100 || loc == 0x200 || loc == 0x300 || loc == 0x400 ||
            loc == 0x500 || loc == 0x600 || loc == 0x700 || loc == 0x800 ||
            loc == 0x900 || loc == 0x1000) {
                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 185660 (usecs)

the difference is bigger now: 484-145 vs 185-145

9 extra 'if' conditions for bpf is almost nothing, since they
translate into 18 new x86 instructions after JITing, but for
interpreter it's obviously costly.

Why 0x100 instead of 0x1? To make sure that compiler doesn't optimize
them into < >
Otherwise it's really really unfair.

ktap is a nice tool. Great job Jovi!
I noticed that it doesn't always clear created kprobes after run and I
see a bunch of .../tracing/events/ktap_kprobes_xxx, but that's a minor
thing.

Thanks
Alexei
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