Messages in this thread | | | Date | Thu, 16 Apr 2009 17:43:40 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH 2/8] tracing: create automated trace defines |
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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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