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