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

[Hemant <hkshaw@linux.vnet.ibm.com>]

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.