KPROBES: Instrumenting a function's call site

From: Avishay Traeger
Date: Tue Sep 25 2007 - 18:13:46 EST

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

So to measure the latency of foo(), I basically want kprobes to do this:

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 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.

Please CC me on any replies (not subscribed to LKML).


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