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

[Masami Hiramatsu]

(2013/12/06 14:19), Jovi Zhangwei wrote:
> Hi Alexei,
> 
> On Thu, Dec 5, 2013 at 12:40 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
>>> On Tue, Dec 3, 2013 at 4:01 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> 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
>>
> There have big differences for compare arg2(in ktap) with direct register
> access(ctx->regs.dx).
> 
> The current argument fetching(arg2 in above testcase) implementation in ktap
> is very inefficient, see ktap/interpreter/lib_kdebug.c:kp_event_getarg.
> The only way to speedup is kernel tracing code change, let external tracing
> module access event field not through list lookup. This work is not
> started yet. :)

I'm not sure why you can't access it directly from ftrace-event buffer.
There is just a packed data structure and it is exposed via debugfs.
You can decode it and can get an offset/size by using libtraceevent.

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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