Date:	Mon, 24 Feb 2014 14:44:41 +0530
From:	Hemant Kumar <hkshaw@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	srikar@...ux.vnet.ibm.com, peterz@...radead.org, oleg@...hat.com,
	hegdevasant@...ux.vnet.ibm.com, mingo@...hat.com, anton@...hat.com,
	systemtap@...rceware.org, namhyung@...nel.org,
	masami.hiramatsu.pt@...achi.com, aravinda@...ux.vnet.ibm.com,
	penberg@....fi
Subject: [RFC PATCH v1 0/2] perf: Support for SDT markers

This patchset helps in listing dtrace style markers(SDT) present in user space
applications through perf.
Notes/markers are placed at important places by the
developers. They have a negligible overhead when not enabled.
We can enable them and probe at these places and find some important information
like the arguments' values, etc.

We have lots of applications which use SDT markers today, like:
Postgresql, MySql, Mozilla, Perl, Python, Java, Ruby, libvirt, QEMU, glib

To add SDT markers into user applications:
We need to have this header sys/sdt.h present.
sys/sdt.h used is version 3.
If not present, install systemtap-sdt-devel package (for fedora-18).

Please refer to the Documentation patch to see how the SDT markers are added into
a program.

With this patchset,
- Use perf to list the markers in the app:
# perf list sdt ./user_app

./user_app :
%user_app:foo_start
%user_app:fun_start

- Also, we can see the SDT markers present in our system in the usual binaries.
These usual binaries are libraries (dsos) listed by ldconfig --print-cache and some
binaries present in PATH environment variable.

First, scan the binaries using :
# perf list sdt --scan

Creating a cache of SDT markers...
perf sdt cache created!
 Use : "perf list sdt"
 to see the SDT markers

After the sdt cache file is created, use perf list to view the markers :
# perf list sdt

%rtld : init_start
%rtld : init_complete
%rtld : map_failed
%rtld : map_start
%rtld : lll_futex_wake
...
...
%libgcc : unwind
%libvirt : rpc_server_client_auth_allow
%libvirt : rpc_server_client_auth_fail
%libvirt : rpc_server_client_auth_deny

Alternatively, one can view the /var/cache/perf/perf-sdt.cache directly.

This link shows an example of marker probing with Systemtap:
https://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps

Also, this link provides important info regarding SDT notes:
http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation

- 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.
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.

Changes since last series :
- Made the SDT markers present in a system more visible through perf list 
  as suggested by Pekka Enberg.
- Less complex command line interface to access the SDT markers.
- Added a cache file to store most of the SDT markers of a system.

TODO:
- Add support to probe these SDT markers and integrate with a previous patch
  (support to perf to probe SDT markers) posted in lkml.
  https://lkml.org/lkml/2013/10/23/10

- Recognizing arguments and support to probe on them.

---

Hemant Kumar (2):
      perf/sdt : Listing of SDT markers by perf
      perf/sdt : Documentation


 tools/perf/Documentation/SDT-markers.txt |  122 +++++++
 tools/perf/Documentation/perf-list.txt   |    8 
 tools/perf/Makefile.perf                 |    1 
 tools/perf/builtin-list.c                |    4 
 tools/perf/util/parse-events.h           |    2 
 tools/perf/util/sdt.c                    |  489 ++++++++++++++++++++++++++++++
 tools/perf/util/symbol-elf.c             |  227 ++++++++++++++
 tools/perf/util/symbol.h                 |   19 +
 8 files changed, 869 insertions(+), 3 deletions(-)
 create mode 100644 tools/perf/Documentation/SDT-markers.txt
 create mode 100644 tools/perf/util/sdt.c

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