A generic tracing API for Linux

Many userspace developers want a "flight recorder" for applications on Linux. While Solaris offers a comprehensive tracing tool, dtrace, Linux is now growing pieces of tracing functionality.

Dynamic kernel tracing remains high on the wishlists presented by many Linux users. While much work has been done to create a powerful tracing capability, very little of that work has found its way into the mainline. The recent posting of one small piece of infrastructure may help to change that situation, though.

The piece in question is the trace layer posted by David Wilder. Its purpose is to make it easy for a tracing application to get things set up in the kernel and allow the user to control the tracing process. To that end, it provides an internal kernel API and a set of control files in the debugfs filesystem.

On the kernel side, a tracing module would set things up with a call to:

 
    #include 

struct trace_info *trace_setup(const char *root, const char *name,

u32 buf_size, u32 buf_nr, u32 flags);

Here, root is the name of the root directory which will appear in debugfs, name is the name of the control directory within root, buf_size and buf_nr describe the size and number of relay buffers to be created, and flags controls various channel options. The TRACE_GLOBAL_CHANNEL flag says that a single set of relay channels (as opposed to per-CPU channels) should be used; TRACE_FLIGHT_CHANNEL turns on the "flight recorder" mode where relay buffer overruns result in the overwriting of old data, and TRACE_DISABLE_STATE disables control of the channel via debugfs.

The return value (if all goes well) will be a pointer to a trace_info structure for the channel. This structure has a number of fields, but the one which will be of most interest outside of the trace code itself will be rchan, which is a pointer to the relay channel associated with this trace point.

When actual tracing is to begin, the kernel module should make a call to:

 
    int trace_start(struct trace_info *trace);
The return value follows the "zero or a negative error value" convention. Tracing is turned off with:
 
    int trace_stop(struct trace_info *trace);
When the tracing module is done, it should shut down the trace with:
 
    void trace_cleanup(struct trace_info *trace);
Note that none of these entry points have anything to do with the placement or activation of trace points or the creation of trace data. All of that must be done separately by the trace module. So a typical module will, after calling trace_start(), set up one or more kprobes or activate a static kernel marker. The probe function attached to the trace points should do something like this:
 
    rcu_read_lock();

if (trace_running(trace)) {

/* Format trace data and output via relay */

}

rcu_read_unlock();

Additionally, if the TRACE_GLOBAL_CHANNEL flag has been set, the probe function should protect access to the relay channel with a spinlock. This protection may also be necessary in situations where an interrupt handler might be traced.

Join the PC World newsletter!

Error: Please check your email address.

Struggling for Christmas presents this year? Check out our Christmas Gift Guide for some top tech suggestions and more.

Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Jonathan Corbet

LinuxWorld

Most Popular Reviews

Follow Us

Best Deals on GoodGearGuide

Shopping.com

Latest News Articles

Resources

GGG Evaluation Team

Kathy Cassidy

STYLISTIC Q702

First impression on unpacking the Q702 test unit was the solid feel and clean, minimalist styling.

Anthony Grifoni

STYLISTIC Q572

For work use, Microsoft Word and Excel programs pre-installed on the device are adequate for preparing short documents.

Steph Mundell

LIFEBOOK UH574

The Fujitsu LifeBook UH574 allowed for great mobility without being obnoxiously heavy or clunky. Its twelve hours of battery life did not disappoint.

Andrew Mitsi

STYLISTIC Q702

The screen was particularly good. It is bright and visible from most angles, however heat is an issue, particularly around the Windows button on the front, and on the back where the battery housing is located.

Simon Harriott

STYLISTIC Q702

My first impression after unboxing the Q702 is that it is a nice looking unit. Styling is somewhat minimalist but very effective. The tablet part, once detached, has a nice weight, and no buttons or switches are located in awkward or intrusive positions.

Latest Jobs

Shopping.com

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?