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

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Date:	Mon, 11 May 2015 18:15:37 +0800
From:	He Kuang <hekuang@...wei.com>
To:	Ingo Molnar <mingo@...nel.org>
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



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.
>
> 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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