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Message-ID: <20070926043933.GA6460@in.ibm.com>
Date: Wed, 26 Sep 2007 10:09:33 +0530
From: Ananth N Mavinakayanahalli <ananth@...ibm.com>
To: Avishay Traeger <atraeger@...sunysb.edu>
Cc: prasanna@...ibm.com, anil.s.keshavamurthy@...el.com,
davem@...emloft.net, linux-kernel@...r.kernel.org
Subject: Re: KPROBES: Instrumenting a function's call site
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.
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