linux-kernel - Re: KPROBES: Instrumenting a function's call site

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Date:	Wed, 26 Sep 2007 22:07:50 +0530
From:	"Abhishek Sagar" <sagar.abhishek@...il.com>
To:	"Avishay Traeger" <atraeger@...sunysb.edu>
Cc:	prasanna@...ibm.com, ananth@...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 9/26/07, Avishay Traeger <atraeger@...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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