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

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Message-ID: <55509A67.1040704@hitachi.com>
Date:	Mon, 11 May 2015 21:02:47 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	He Kuang <hekuang@...wei.com>, Ingo Molnar <mingo@...nel.org>
CC:	a.p.zijlstra@...llo.nl, acme@...nel.org, jolsa@...nel.org,
	mingo@...hat.com, wangnan0@...wei.com, linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH v3 3/3] perf probe: Show better error message when
 failed to find variable

On 2015/05/11 19:15, He Kuang wrote:
> 
> 
> On 2015/5/11 17:50, Ingo Molnar wrote:
>> * 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.

OK, He, could you also include this fix?

Thank you,


>>
>> Another question: what does 'generic_perform_write+118' mean here? Why
>> the offset?
> 
> This is for setting a kprobe at the location has an offset of 118
> bytes to the entry of the function 'generic_perform_write'.
> 
> In the previous patch: https://lkml.org/lkml/2015/5/11/170,
> we show a valid byte offset range of variables in scope, 118 is
> an invalid address which can be read from the result, this offset
> is just for showing the error message.
> 
>>
>> Thanks,
>>
>> 	Ingo
>> --
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>>
> 
> 
> 


-- 
Masami HIRAMATSU
Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
E-mail: masami.hiramatsu.pt@...achi.com
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