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    SubjectRe: [PATCH 2/8] tracing: create automated trace defines
    Mathieu Desnoyers wrote:
    > "all this code" is actually :
    >
    > rcu_read_lock_sched_notrace(); \
    > it_func = rcu_dereference((tp)->funcs); \
    > if (it_func) { \
    > do { \
    > ((void(*)(proto))(*it_func))(args); \
    > } while (*(++it_func)); \
    > } \
    > rcu_read_unlock_sched_notrace(); \
    >
    > Which does nothing more than disabling preemption and a for loop to
    > call all the tracepoint handlers. I don't see the big win in laying out
    > the stack to call this code out-of-line; we would just remove the
    > preempt disable and the loop, which are minimal compared to most
    > call stacks.
    >

    Well, look at it from my perspective: Ingo has been repeatedly beating
    me up for the overhead pvops adds to a native kernel, where it really is
    just a (direct) function call. I want to instrument each pvop site with
    a tracepoint so I can actually work out which calls are being called how
    frequently to look for new optimisation opportunities.

    I would guess the tracepoint code sequence is going to increase the
    impact of each pvop call site by a fair bit, and that's not counting the
    effects the extra register pressure will have. That's a pile of code to
    add.

    And frankly, that's fine by me, because I would expect this degree of
    introspection to have some performance hit. But it does make the need
    for per-subsystem tracing Kconfig entries fairly important, because I
    don't think this would be acceptable to ship in a non-debug-everything
    kernel build, even though other tracepoints might be.

    > So basically, tracepoints are already just doing a function call, with a
    > few more bytes for preempt disable and multiple handler support.
    >
    > About the compiler deciding to put the unlikely branch out-of-line, I've
    > never seen any function calls generated just for the sake of saving
    > those few bytes, that would be crazy of the part of the compiler.
    > However, it can (and should) freely put the stack setup in the coldest
    > cache-lines possible, which are reachable by a near jump.
    >

    No, it wouldn't generate a call. But if its going to put the code out
    of line into cold cache-lines, then it may as well generate a call.

    Anyway, the important point from my perspective is that tracepoint.h
    have no #include dependencies beyond linux/types.h (compiler.h, etc).

    J


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