Re: Re: [RFC PATCH 0/2] Perf support to SDT markers

[Masami Hiramatsu]

(2013/09/04 15:49), Namhyung Kim wrote:
> On Tue, 03 Sep 2013 18:53:17 +0530, Hemant wrote:
>> On 09/03/2013 02:47 PM, Masami Hiramatsu wrote:
>>> (2013/09/03 17:25), Ingo Molnar wrote:
>>>> * Hemant Kumar Shaw <hkshaw@xxxxxxxxxxxxxxxxxx> wrote:
>>>>
>>>>> This series adds support to perf to list and probe into the SDT markers.
>>>>> The first patch implements listing of all the SDT markers present in
>>>>> the ELFs (executables or libraries). The SDT markers are present in the
>>>>> .note.stapsdt section of the elf. That section can be traversed to list
>>>>> all the markers. Recognition of markers follows the SystemTap approach.
>>>>>
>>>>> The second patch will allow perf to probe into these markers. This is
>>>>> done by writing the marker name and its offset into the
>>>>> uprobe_events file in the tracing directory.
>>>>> Then, perf tools can be used to analyze perf.data file.
>>>> Please provide a better high level description that explains the history
>>>> and scope of SDT markers, how SDT markers get into binaries, how they can
>>>> be used for probing, a real-life usage example that shows something
>>>> interesting not possible via other ways, etc.
>>> Indeed, and also I'd like to know what versions of SDT this support,
>>> and where we can see the technical document of that. As far as I know,
>>> the previous(?) SDT implementation also involves ugly semaphores.
>>> Have that already gone?
> 
> It seems it's not. I see the SDT v3 document still mentions semaphores.

Uh, how the current implementation can avoid those semaphores?
(It seems that this patch doesn't mention it.)

I had discussed that the semaphore problem 3 years ago with systemtap
guys, it should be there to keep the main process away from "heavy"
argument processing when the marker is disabled, especially for python vm.
If those are still there, we can't enable markers without tweaking the
"semaphore" by using systemtap or a debugger.

>> This link shows an example of marker probing with Systemtap:
>> https://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps
> 
> I think the link below would be more helpful for us :)
> 
> http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation

Thank you :)


-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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