Messages in this thread | | | Date | Thu, 30 Oct 2008 19:20:56 +0100 | From | Ingo Molnar <> | Subject | Re: [RFC][PATCH] Make ftrace able to trace function return |
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* 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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