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Systemtap Static Probes

Summary

Systemtap allows event tracing of programs when they have static probes inserted. This allows for tracing specifics of an application on a higher level that is meaningful to the application user so they don't have to know the exact source code details for tracing what is happening.

Owner

  • email: mjw@redhat.com

Current status

TODO

  • Some experiments showed some greater than expected impact on performance in the dormant case. So for each package identified we need to do some benchmarking to make sure there is zero overhead in to normal case. (Add any relevants benchmarks to the scopes section under the relevant package please).
  • Identified deficiencies upstream:
 * [PR10013] support ENABLED sdt probe macro
 * [PR10601] user-space deref/registers in loc2c (i386 mainly, but could affect x86_64 and other arches also)

Detailed Description

By packaging a new version of systemtap, that enables programs that already have static dtrace probe markers in their sources and by making those packages build depend on the new systemtap-sdt-devel package and recompiling them with probe points enabled, users of those packages will be able to trace any high level events that these packages provide.

Benefit to Fedora

It will be easier for developers and users to observe what is really happening on their system on a higher (application) level.

Scope

  • Work with upstream to identify any issues with the new capabilities while we activate probes in packages.
  • Package new version of Systemtap (including new subpackage systemtap-sdt-devel).
  • Identify packages that already include static user probes (see below)
  • Work with package maintainer to enable them in the Fedora build spec file.
  • Add documentation on enabled probes and how to use them with a systemtap tapset.

Currently identified packages:

postgresql

Tracking bug: https://bugzilla.redhat.com/show_bug.cgi?id=488941 Already able to build something that works with current rpm. Documentation: upstream docs Example: example trace Screencast: video presentation

xorg-x11-server

Need tweaks to systemtap to gen proper header from .d file. (upstream docs) - stan and/or rajan

java-1.6.0-openjdk

Tracking bug: https://bugzilla.redhat.com/show_bug.cgi?id=498109 Probably likewise, has .d files in there. (upstream docs) - stan and/or mjw Static probes ready, plus hotspot tapset, jni tapset and java backtraces pending upstream.

tcl

Tracking bug: https://bugzilla.redhat.com/show_bug.cgi?id=489017

  • tcl-8.4.16+: Has a single generic/tclDTrace.d file. (upstream docs) - stan

Might need implementation of probe_ENABLED(), which is tracked upstream as http://sourceware.org/bugzilla/show_bug.cgi?id=10013

Firefox

Tracking bug: https://bugzilla.redhat.com/show_bug.cgi?id=496113

mysql

  • mysql 6.0.8: really new alpha version from mysql.com has probes. However, the version in fedora 5.0.67 doesn't. A backport would be required. Won't be in next fedora

apachetop/httpd

  • apachetop-0.12.6 (newer httpd have patches directly [1] - note needs some build tweaking)

Perl

perl-5.10.0: implemented as an out-of-tree patch to the core (5.10.1 has some, see INSTALL)

Python

Ideas for python probes:

  • function calls/returns (this is what the dtrace probe has)
  • GIL events: instrument the raw function to claim/release of the lock, then capture the times at which it happens, then render stats
  • threads starting/stopping
  • bytecode execution metrics: e.g. trace individual bytecodes; how often does LOAD_GLOBAL get invoked
  • exceptions being thrown
  • unhandled exceptions
  • arenas being claimed/freed
  • dictionaries switching to inefficient form: http://lewk.org/blog/python-dictionary-optimizations
  • _warnings.c: do_warn() (e.g. whole-system python3 warnings for all python 2 running on your system)
  • py-level backtraces

PHP

  • php-5.2.8: implemented as an add-on module (shared library listed in /etc/php.ini) that intercepts internal php interpreter function pointers and wraps those calls with markers

Ruby

  • ruby-1.8.6-p287: similar to php, but richer & far more complicated

See http://ruby-dtrace.rubyforge.org/

Haskell

Notes

It seems as if several of the above were dtrace-instrumented in code that was never merged into the upstream versions of the package, but instead represented as run-time add-ons or private patches for Solaris distributions. Disappointing, but perhaps we can do better and engage the respective upstream teams. This will of course take time and panache.

At least the patches tend to be very small so we have some freedom to choose between approaches (adding STAP_PROBE/whatever hooks directly to the core upstream code; or fedora local patches; or add-on shared libraries like for php/httpd).

Another approach worth considering is adding tapsets that map process.mark() events to process.function/statement() to approximate the dtrace out-of-tree patches.

How To Test

Whether systemtap and static markers are working in general can be tested by installing systemtap, kernel-debuginfo and the systemtap-testsuite. Running sudo make installcheck in /usr/share/systemtap/testsuite

When applications get static markers enabled we should add them to a testing page listing:

  • Package install instructions.
  • Setup and sample run of the application
  • A reference to the probe names.
  • And an simple example stap invocation listing markers that can be enabled.

Question: Is there a convention/template for adding such test pages for test days?
Answer: https://fedoraproject.org/wiki/QA/Test_Days/Create

User Experience

For packages that have static probes enabled users will be able to trace high-level events, like for example database transactions, through stap.

Dependencies

  • A new version of systemtap with the systemtap-sdt-devel subpackage.
  • Any package wishing to expose existing probes in its (upstream) sources depending on systemtap-sdt-devel and adding an --enable-dtrace or equivalent to its spec file.

Contingency Plan

Even if all the tracing will not work, packages that are converted to provide static probes will not be impacted since the probe points have (near) zero overhead, so in the worse case some packages were recompiled to enable the feature, but users will still not be able to use it.

Documentation

The upstream wiki is the best description for now http://sourceware.org/systemtap/wiki/UsingStaticUserMarkers the systemtap list has an example on converting a package http://sourceware.org/ml/systemtap/2009-q1/msg00140.html

While working on this feature this section will be expanded to list packages that have probe points enabled and pointers to (upstream) package documentation on the probe names and semantics like for postgresql http://www.postgresql.org/docs/8.2/static/dynamic-trace.html

Release Notes

Systemtap has been extended to support user space tracing, and in particular to support static (dtrace compatible) markers enabled in various programs in Fedora 12. This enables users, developers and administrators a high level overview of what is going on with their system or deep down in a specific program or subsystem.

Systemtap comes with a tutorial, a language reference manual, a tapsets reference and an examples directory under /usr/share/doc/systemtap-?.?/

  • TODO: Should have a list of which packages were enabled with markers when finished.

Comments and Discussion


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