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Re: [RFC PATCH 0/2] Perf support to SDT markers
- From: Namhyung Kim <namhyung at kernel dot org>
- To: Hemant <hkshaw at linux dot vnet dot ibm dot com>
- Cc: Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>, Ingo Molnar <mingo at kernel dot org>, linux-kernel at vger dot kernel dot org, srikar at linux dot vnet dot ibm dot com, peterz at infradead dot org, oleg at redhat dot com, mingo at redhat dot com, anton at redhat dot com, systemtap at sourceware dot org
- Date: Wed, 04 Sep 2013 15:49:24 +0900
- Subject: Re: [RFC PATCH 0/2] Perf support to SDT markers
- Authentication-results: sourceware.org; auth=none
- References: <20130903072944 dot 4793 dot 93584 dot stgit at hemant-fedora> <20130903082503 dot GA20732 at gmail dot com> <5225A937 dot 2050507 at hitachi dot com> <5225E2C5 dot 3080001 at linux dot vnet dot ibm dot com>
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.
> 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.