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Message-ID: <863e9df20709260937v79d555ffn49f3b8ff7aba42cb@mail.gmail.com>
Date: Wed, 26 Sep 2007 22:07:50 +0530
From: "Abhishek Sagar" <sagar.abhishek@gmail.com>
To: "Avishay Traeger" <atraeger@cs.sunysb.edu>
Subject: Re: KPROBES: Instrumenting a function's call site
Cc: prasanna@in.ibm.com, ananth@in.ibm.com, anil.s.keshavamurthy@intel.com,
       davem@davemloft.net, linux-kernel@vger.kernel.org
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On 9/26/07, Avishay Traeger <atraeger@cs.sunysb.edu> wrote:
> 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.

Single-stepping is done with preemption (and on some archs like ARM,
interrupts) disabled. May be that is contributing to such a skew.

> 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).
>
> Thanks,
> Avishay
>
--
Regards
Abhishek Sagar

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