[PATCH v2 0/3] Perf support to SDT markers

[Hemant Kumar]

This patchset helps in probing dtrace style markers(SDT) present in user space
applications through perf. Notes/markes 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.

How 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).

A simple example to show this follows.
- Create a file with .d extension and mention the probe names in it with
provider name and marker name.

$ cat probes.d
provider user_app {
         probe foo_start();
	    probe fun_start();                                                   
};                                                                            

- Now create the probes.h and probes.o file :
$ dtrace -C -h -s probes.d -o probes.h
$ dtrace -C -G -s probes.d -o probes.o

- A program using the markers:

$ cat user_app.c

#include <stdio.h>
#include "probes.h"

void foo(void)
{
	USER_APP_FOO_START();
	printf("This is foo\n");                                              
}

void fun(void)
{
	USER_APP_FUN_START();
	printf("Inside fun\n");                                                       
}
int main(void)
{
	printf("In main\n");
	foo();
	fun();                                                                      
	return 0;                                                                   
}

- Compile it and also provide probes.o file to linker:
$ gcc user_app.c probes.o -o user_app

- Now use perf to list the markers in the app:
# perf probe --markers -x ./user_app

%user_app:foo_start
%user_app:fun_start

- And  then use perf probe to add a probe point :

# perf probe -x ./user_app -a '%user_app:foo_start'

Added new event :
event = foo_start      (on 0x530)

You can now use it on all perf tools such as :

     perf record -e probe_user:foo_start -aR sleep 1

# perf record -e probe_user:foo_start -aR ./user_app
In main
This is foo
Inside fun
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.235 MB perf.data (~10279 samples) ]

- Then use perf tools to analyze it.
# perf report --stdio

# ========
# captured on: Tue Sep  3 16:19:55 2013
# hostname : hemant-fedora
# os release : 3.11.0-rc3+
# perf version : 3.9.4-200.fc18.x86_64
# arch : x86_64
# nrcpus online : 2
# nrcpus avail : 2
# cpudesc : QEMU Virtual CPU version 1.2.2
# cpuid : GenuineIntel,6,2,3
# total memory : 2051912 kBIf these are not enabled, they are present in the \
ELF as nop.

# cmdline : /usr/bin/perf record -e probe_user:foo_start -aR ./user_app
# event : name = probe_user:foo_start, type = 2, config = 0x38e, config1
= 0x0, config2 = 0x0, excl_usr = 0, excl_kern = 0, excl_host = 0,
excl_guest = 1, precise_ip = 0
# HEADER_CPU_TOPOLOGY info available, use -I to display
# HEADER_NUMA_TOPOLOGY info available, use -I to display
# pmu mappings: software = 1, tracepoint = 2, breakpoint = 5
# ========
#
# Samples: 1  of event 'probe_user:foo_start'
# Event count (approx.): 1
#
# Overhead   Command  Shared Object   Symbol
# ........  ........  .............  .......
#
   100.00%  user_app  user_app       [.] foo


#
# (For a higher level overview, try: perf report --sort comm,dso)
#

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 are stored.
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 v1:
- Made some structural changes.
- Changed the option required to list/probe into SDT notes.
- Unified function names.
- Added some necessary checks.
- Ignored semaphore enabled SDT notes.
- Added documentation.
- Removed some redundancies.

TODO:
- Recognizing SDT notes' arguments and support to probe on them.
---

Hemant Kumar (3):
      SDT markers listing by perf:
      Support for perf to probe into SDT markers:
      Documentation regarding perf/sdt


 tools/perf/Documentation/perf-probe.txt |   15 ++
 tools/perf/Documentation/sdt-probes.txt |  163 ++++++++++++++++++++
 tools/perf/builtin-probe.c              |   35 ++++
 tools/perf/util/probe-event.c           |  128 +++++++++++++++-
 tools/perf/util/probe-event.h           |    4 
 tools/perf/util/symbol-elf.c            |  256 +++++++++++++++++++++++++++++++
 tools/perf/util/symbol.h                |   21 +++
 7 files changed, 610 insertions(+), 12 deletions(-)
 create mode 100644 tools/perf/Documentation/sdt-probes.txt

-- 

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