Messages in this thread | | | Date | Wed, 26 Sep 2007 10:09:33 +0530 | From | Ananth N Mavinakayanahalli <> | Subject | Re: KPROBES: Instrumenting a function's call site |
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On Tue, Sep 25, 2007 at 06:12:38PM -0400, Avishay Traeger wrote: > Hello, > I am trying to use kprobes to measure the latency of a function by > instrumenting its call site. Basically, I find the call instruction, > and insert a kprobe with a pre-handler and post-handler at that point. > The pre-handler measures the latency (reads the TSC counter). The > post-handler measures the latency again, and subtracts the value that > was read in the pre-handler to compute the total latency of the called > function.
This sounds ok...
> So to measure the latency of foo(), I basically want kprobes to do this: > pre_handler(); > foo(); > post_handler(); > > The problem is that the latencies that I am getting are consistently low > (~10,000 cycles). When I manually instrument the functions, the latency > is about 20,000,000 cycles. Clearly something is not right here.
Is foo() called from too many different places? If so, are you interested with only the invocation of foo() from a specific callsite?
> Is this a known issue? Instead of using the post-handler, I can try to > add a kprobe to the following instruction with a pre-handler. I was > just curious if there was something fundamentally wrong with the > approach I took, or maybe a bug that you should be made aware of.
I am not too sure... single-stepping a "call" instruction from a different memory location (single-stepping out of line) requires some fixups and kprobes handles such fixups just fine (see resume_execution() in arch/<arch>/kernel/kprobes.c)
You could try a a couple of approaches for starters.
a. As you mention above, a kprobe on the function invocation and the other on the instruction following the call; both need just pre_handlers.
b. - Insert a kprobe and a kretprobe on foo() - The kprobe needs to have only a pre_handler that'll measure the latency - A similar handler for the kretprobe handler can measure the latency again and their difference will give you foo()'s latency.
<b> though will require you to do some housekeeping in case foo() is reentrant to track which return instance corresponds to which call.
Ananth
PS: There was a thought of providing a facility to run a handler at function entry even when just a kretprobe is used. Maybe we need to relook at that; it'd have been useful in this case. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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