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    SubjectRe: [RFC][PATCH] Make ftrace able to trace function return

    * Frederic Weisbecker <fweisbec@gmail.com> wrote:

    > Hi.
    >
    > First of all, I want to say this patch is absolutely not ready for
    > inclusion. Some parts are really ugly and the thing is only
    > partially functionning.
    >
    > It's just an idea or a kind of proof of concept. I just wanted to
    > make ftrace able to measure the time of execution of a function. For
    > that I had to hook both the function call and its return.
    >
    > By using mcount, we can hook the function on enter and we can
    > override its return address. So we can catch the time at those two
    > points. The problem comes when a function run concurrently through
    > preemption or smp. We can measure the return time but how to be sure
    > which time capture we had on call since this time could have been
    > captured multiple times. And for the same reason, how to make sure
    > of the return address.
    >
    > So the idea is to allocate a general set of slots on which we can
    > save our original return address and the call time. After that we
    > change the return address of the hooked function to jump on a
    > trampoline which will push the offset for us to retrieve the slot on
    > the set for this function call. Then the trampoline will call a
    > return handler that will trace the return time and send all of these
    > informations to a specific tracer. And then the return handler will
    > return to the original return address.
    >
    > To determine quickly which slot is free, I use a bitmap of 32 bits.
    > Perhaps it is a bad assumption but I could enlarge it and there is
    > an overrun counter. This is the only point which needs to be
    > protected against concurrent access.
    >
    > I made a tracer for this but the problem is that the capture by
    > ftrace will hang the system if we can use several slots. When I
    > dedicate only one free slot, wherever on the set, there is no
    > problem but I miss a lot of calls. So by default on this patch,
    > there is only one slot dedicated on the bitmap.
    >
    > Don't hesitate to comment this patch made of trashes...

    hm, are you aware of the -finstrument-functions feature of GCC?

    that feature generates such entry points at build time:

    void __cyg_profile_func_enter (void *this_fn,
    void *call_site);
    void __cyg_profile_func_exit (void *this_fn,
    void *call_site);

    this might be faster/cleaner than using a trampoline approach IMO.

    OTOH, entry+exit profiling has about double the cost of just entry
    profiling - so maybe there should be some runtime flexibility there.
    Plus the same recordmcount trick should be used to patch up these
    entry points to NOP by default.

    Ingo


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