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

[Alexei Starovoitov]

On Fri, Dec 6, 2013 at 3:54 PM, Masami Hiramatsu
<masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
> (2013/12/06 14:16), Alexei Starovoitov wrote:
>> On Thu, Dec 5, 2013 at 5:20 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> 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. :)

it is indeed very cool.
Thanks!
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