linux-kernel - Re: [PATCH v3 3/3] perf probe: Show better error message when failed to find variable

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Message-ID: <20150511095052.GA7441@gmail.com>
Date:	Mon, 11 May 2015 11:50:52 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	He Kuang <hekuang@...wei.com>
Cc:	masami.hiramatsu.pt@...achi.com, a.p.zijlstra@...llo.nl,
	acme@...nel.org, jolsa@...nel.org, mingo@...hat.com,
	wangnan0@...wei.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] perf probe: Show better error message when failed
 to find variable


* He Kuang <hekuang@...wei.com> wrote:

> Hi, Ingo
> 
> On 2015/5/11 17:30, Ingo Molnar wrote:
> >* He Kuang <hekuang@...wei.com> wrote:
> >
> >>Indicate to check variable location range in error message when we got
> >>failed to find the variable.
> >>
> >>Before this patch:
> >>
> >>   $ perf probe --add 'generic_perform_write+118 bytes'
> >>   Failed to find the location of bytes at this address.
> >>    Perhaps, it has been optimized out.
> >>     Error: Failed to add events.
> >>
> >>After this patch:
> >>   $ perf probe --add 'generic_perform_write+118 bytes'
> >>   Failed to find the location of bytes at this address.
> >What does this sentence mean? I thought 'address' means 'location of
> >bytes'. So the address identifies the location and obviously we know
> >that. So this message wants to say something else.
> 
> 'generic_perform_write' is a function name, while 'bytes' is a local
> variable in this function. Maybe the variable I chose make you confused.
> 
> This maybe clear:
>   Failed to find the location of 'bytes' at this address.

Yeah, absolutely! This highlights the importance of putting 
user-supplied symbols into quotes and such.

Maybe even write:

    Failed to find the location of the 'bytes' variable at this address.

Another question: what does 'generic_perform_write+118' mean here? Why 
the offset?

Thanks,

	Ingo
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