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

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2013/12/06 14:16), Alexei Starovoitov wrote:
> On Thu, Dec 5, 2013 at 5:20 PM, Andi Kleen <andi@firstfloor.org> wrote:
>>> the difference is bigger now: 484-145 vs 185-145
>>
>> This is a obvious improvement, but imho not big enough to be extremely
>> compelling (< cost 1-2 cache misses, no orders of magnitude improvements
>> that would justify a lot of code)
> 
> hmm. we're comparing against ktap here…
> which has 5x more kernel code and 8x slower in this test...
> 
>> Your code requires a compiler, so from my perspective it
>> wouldn't be a lot easier or faster to use than just changing
>> the code directly and recompile.
>>
>> The users want something simple too that shields them from
>> having to learn all the internals. They don't want to recompile.
>> As far as I can tell your code is a bit too low level for that,
>> and the requirement for the compiler may also scare them.
>>
>> Where exactly does it fit?
> 
> the goal is to have llvm compiler next to perf, wrapped in a user friendly way.
> 
> compiling small filter vs recompiling full kernel…
> inserting into live kernel vs rebooting …
> not sure how you're saying it's equivalent.
> 
> In my kernel debugging experience current tools (tracing, systemtap)
> were rarely enough.
> I always had to add my own printks through the code, recompile and reboot.
> Often just to see that it's not the place where I want to print things
> or it's too verbose.
> Then I would adjust printks, recompile and reboot again.
> That was slow and tedious, since I would be crashing things from time to time
> just because skb doesn't always have a valid dev or I made a typo.
> For debugging I do really need something quick and dirty that lets me
> add my own printk
> of whatever structs I want anywhere in the kernel without crashing it.
> That's exactly what bpf tracing filters do.

I recommend you to use perf-probe. That will give you an easy solution. :)


Thank you,

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