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


On 09/04/2013 12:19 PM, 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@linux.vnet.ibm.com> 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.

Yes, It still mentions semaphores and I will be handling these semaphores.

Thanks
Hemant


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

Thanks,
Namhyung

- Markers in binaries :
These SDT markers are present in the ELF in the section named
".note.stapsdt".
Here, the name of the marker, its provider, type, location, base
address, semaphore address, arguments are present.
We can retrieve these values using the members name_off and desc_off in
Nhdr structure. If these are not enabled, they are present in the ELF
as nop.


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