Tracing and Profiling: Difference between revisions

Revision as of 21:24, 16 October 2012

Tracing and Profiling in Yocto

Yocto bundles a number of tracing and profiling tools - this 'HOWTO' describes their basic usage and more importantly shows by example how they fit together and how to make use of them to solve real-world problems.

The tools presented are for the most part completely open-ended and have quite good and/or extensive documentation of their own which can be used to solve just about any problem you might come across in Linux. Each section that describes a particular tool has links to that tool's documentation and website.

The purpose of this 'HOWTO' is to present a set of common and generally useful tracing and profiling idioms along with their application (as appropriate) to each tool, in the context of a general-purpose 'drill-down' methodology that can be applied to solving a large number (90%?) of problems. For help with more advanced usages and problems, please see the documentation and/or websites listed for each tool.

General Setup

Most of the tools are available only in 'sdk' images or in images built after adding 'tools-profile' to your local.conf. So, in order to be able to access all of the tools described here, please first build and boot an 'sdk' image e.g.

$ bitbake core-image-sato-sdk

or alternatively by adding 'tools-profile' to the EXTRA_IMAGE_FEATURES line in your local.conf:

EXTRA_IMAGE_FEATURES = "debug-tweaks tools-profile"

If you use the 'tools-profile' method, you don't need to build an sdk image - the tracing and profiling tools will be included in non-sdk images as well e.g.:

$ bitbake core-image-sato

Overall Architecture of the Linux Tracing and Profiling Tools

It may seem surprising to see a section covering an 'overall architecture' for what seems to be a random collection of tracing tools that together make up the Linux tracing and profiling space. The fact is, however, that in recent years this seemingly disparate set of tools has started to converge on a 'core' set of underlying mechanisms:

static tracepoints
dynamic tracepoints
- kprobes
- uprobes
the perf_events subsystem
debugfs

A Few Real-world Examples

Custom Top

Yocto Bug 3049

Slow write speed on live images with denzil

Autodidacting the Graphics Stack

Using ftrace, perf, and systemtap to learn about the i915 graphics stack.

Determining whether 3-D rendering is using the hardware (without special test-suites)

The standard (simple) 3-D graphics programs can't always be used to unequivocally determine whether hardware rendering or a fallback software rendering mode is being used e.g. PVR graphics. We can however use the tracing tools to unequivocally determine whether hardware or software rendering is being used regardless of what the test programs are telling us, or in spite of the fact that we may be using a proprietary stack.

This example will provide a simple yes/no test based on tracing output.

Basic Usage (with examples) for each of the Yocto Tracing Tools

perf

ftrace

trace-cmd/kernelshark

oprofile

sysprof

LTTng (Linux Trace Toolkit, next generation)

Setup

NOTE: The lttng support in Yocto 1.3 (danny) needs the following poky commits applied in order to work:

If you also want to view the LTTng traces graphically, you also need to download and install/run the 'SR1' or later Juno release of eclipse e.g.:

http://www.eclipse.org/downloads/download.php?file=/technology/epp/downloads/release/juno/SR1/eclipse-cpp-juno-SR1-linux-gtk-x86_64.tar.gz

Collecting and Viewing a Trace in Eclipse

Once you've applied the above commits and built and booted your image (you need to build the core-image-sato-sdk image or the other methods described in the General Setup section), you're ready to start tracing.

First, start eclipse and open the 'LTTng Kernel' perspective by selecting the following menu item:

Window | Open Perspective | Other...

In the dialog box that opens, select 'LTTng Kernel' from the list.

Back at the main menu, select the following menu item:

File | New | Project...

In the dialog box that opens, select the 'Tracing | Tracing Project' wizard and press 'Next>'.

Give the project a name and press 'Finish'.

That should result in an entry in the 'Project' subwindow.

In the 'Control' subwindow just below it, press 'New Connection'.

Add a new connection, giving it the hostname or IP address of the target system.

Also provide the username and password of a qualified user (a member of the 'tracing' group) or root account on the target system.

Also, provide appropriate answers to whatever else is asked for e.g. 'secure storage password' can be anything you want

blktrace

blktrace is a tool for tracing and reporting low-level disk I/O. blktrace provides the tracing half of the equation; its output can be piped into the blkparse program, which renders the data in a human-readable form and does some basic analysis:

$ blktrace /dev/sda -o - | blkparse -i -

systemtap

SystemTap is a system-wide script-based tracing and profiling tool.

SystemTap scripts are C-like programs that are executed in the kernel to gather/print/aggregate data extracted from the context they end up being invoked under.

For example, this probe from the SystemTap tutorial [1] simply prints a line every time any process on the system open()s a file. For each line, it prints the executable name of the program that opened the file, along with its pid, and the name of the file it opened (or tried to open), which it extracts from the open syscall's argstr.

probe syscall.open
{
        printf ("%s(%d) open (%s)\n", execname(), pid(), argstr)
}

probe timer.ms(4000) # after 4 seconds
{
        exit ()
}

Normally, to execute this probe, you'd simply install systemtap on the system you want to probe, and directly run the probe on that system e.g. assuming the name of the file containing the above text is trace_open.stp:

# stap trace_open.stp

What systemtap does under the covers to run this probe is 1) parse and convert the probe to an equivalent 'C' form, 2) compile the 'C' form into a kernel module, 3) insert the module into the kernel, which arms it, and 4) collect the data generated by the probe and display it to the user.

In order to accomplish steps 1 and 2, the 'stap' program needs access to the kernel build system that produced the kernel that the probed system is running. In the case of a typical embedded system (the 'target'), the kernel build system unfortunately isn't typically part of the image running on the target. It is normally available on the 'host' system that produced the target image however; in such cases, steps 1 and 2 are executed on the host system, and steps 3 and 4 are executed on the target system, using only the systemtap 'runtime'.

The systemtap support in Yocto assumes that only steps 3 and 4 are run on the target; it is possible to do everything on the target, but this section assumes only the typical embedded use-case.

So basically what you need to do in order to run a systemtap script on the target is to 1) on the host system, compile the probe into a kernel module that makes sense to the target, 2) copy the module onto the target system and 3) insert the module into the target kernel, which arms it, and 4) collect the data generated by the probe and display it to the user.

Unfortunately, the process detailed below isn't as simple as 'stap script.stp', but I have created a simple script that does simplify usage quite a bit (see the 'crosstap' script below).

$ cd /path/to/yocto
$ source oe-init-build-env 

### Shell environment set up for builds. ###

You can now run 'bitbake <target>'

Common targets are:
   core-image-minimal
   core-image-sato
   meta-toolchain
   meta-toolchain-sdk
   adt-installer
   meta-ide-support

You can also run generated qemu images with a command like 'runqemu qemux86'

Once you've done that, you can cd to whatever directory contains your scripts and use 'crosstap' to run the script:

$ cd /path/to/my/systemap/script
$ crosstap root@192.168.7.2 trace_open.stp

If you get an error connecting to the target e.g.:

$ crosstap root@192.168.7.2 trace_open.stp 
error establishing ssh connection on remote 'root@192.168.7.2'

Try ssh'ing to the target and see what happens:

$ ssh root@192.168.7.2

A lot of the time, connection problems are due specifying a wrong IP address or having a 'host key verification error'.

If everything worked as planned, you should see something like this (enter the password when prompted, or press enter if its set up to use no password):

$ crosstap root@192.168.7.2 trace_open.stp 
root@192.168.7.2's password: 
matchbox-termin(1036) open ("/tmp/vte3FS2LW", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE, 0600)
matchbox-termin(1036) open ("/tmp/vteJMC7LW", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE, 0600)