linux-kernel - Re: [QUESTION] Can uprobe_event support @ADDR, $retval, offs(FETCHARG)?

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Date:	Wed, 26 Sep 2012 16:07:34 +0900
From:	Hyeoncheol Lee <hyc.lee@...il.com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linux-kernel@...r.kernel.org, yrl.pp-manager.tt@...achi.com
Subject: Re: [QUESTION] Can uprobe_event support @ADDR, $retval, offs(FETCHARG)?

2012/9/26 Srikar Dronamraju <srikar@...ux.vnet.ibm.com>:
>>
>> Perhaps, it is not so small things, but at least, we can try.
>> In the userspace, memories(pages) can be paged out on swap or
>> files. In that case, memory dereference function needs to track
>> down the data on the disk and it causes I/O. This means we will
>> see the visible performance degradation with tracing.
>> And also, sometime a pointer value (address) is broken, in that
>> case we have to ensure the address is actually valid before
>> accessing it.
>>
>> Of cause, without tracking paged-out data, it is easy
>> to support, because that is already done in kprobe event.
>> I'm not sure how it is useful, because sometimes it will
>> fail to access gather the data.
>> However it is good for the first step, I think.
>>
>> Srikar, what would you think?
>
> I think we should do the best effort basis first. i.e support for
> tracking data thats not paged out.
> Most times the data that is requested tends to the hot data.
>
> We could look at supporting data that is paged out later.
>
>>
>> BTW, if we can support offs(FETCHARGS), $stack and $stackN
>> are also available. ;)
>>
>
> --
> Thanks and Regards
> Srikar Dronamraju
>

I asked this question. because I need Perf that supports to access
user process's variables. and I am trying to add this feature.

Thank you for your answer.

Hyeoncheol
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