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![Steven Rostedt (Red Hat)](/assets/img/avatar_default.png)
There are some code paths in the kernel that need to do some preparations before it calls a tracepoint. As that code is worthless overhead when the tracepoint is not enabled, it would be prudent to have that code only run when the tracepoint is active. To accomplish this, all tracepoints now get a static inline function called "trace_<tracepoint-name>_enabled()" which returns true when the tracepoint is enabled and false otherwise. As an added bonus, that function uses the static_key of the tracepoint such that no branch is needed. if (trace_mytracepoint_enabled()) { arg = process_tp_arg(); trace_mytracepoint(arg); } Will keep the "process_tp_arg()" (which may be expensive to run) from being executed when the tracepoint isn't enabled. It's best to encapsulate the tracepoint itself in the if statement just to keep races. For example, if you had: if (trace_mytracepoint_enabled()) arg = process_tp_arg(); trace_mytracepoint(arg); There's a chance that the tracepoint could be enabled just after the if statement, and arg will be undefined when calling the tracepoint. Link: http://lkml.kernel.org/r/20140506094407.507b6435@gandalf.local.home Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
145 lines
5.1 KiB
Text
145 lines
5.1 KiB
Text
Using the Linux Kernel Tracepoints
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Mathieu Desnoyers
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This document introduces Linux Kernel Tracepoints and their use. It
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provides examples of how to insert tracepoints in the kernel and
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connect probe functions to them and provides some examples of probe
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functions.
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* Purpose of tracepoints
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A tracepoint placed in code provides a hook to call a function (probe)
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that you can provide at runtime. A tracepoint can be "on" (a probe is
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connected to it) or "off" (no probe is attached). When a tracepoint is
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"off" it has no effect, except for adding a tiny time penalty
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(checking a condition for a branch) and space penalty (adding a few
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bytes for the function call at the end of the instrumented function
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and adds a data structure in a separate section). When a tracepoint
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is "on", the function you provide is called each time the tracepoint
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is executed, in the execution context of the caller. When the function
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provided ends its execution, it returns to the caller (continuing from
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the tracepoint site).
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You can put tracepoints at important locations in the code. They are
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lightweight hooks that can pass an arbitrary number of parameters,
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which prototypes are described in a tracepoint declaration placed in a
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header file.
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They can be used for tracing and performance accounting.
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* Usage
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Two elements are required for tracepoints :
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- A tracepoint definition, placed in a header file.
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- The tracepoint statement, in C code.
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In order to use tracepoints, you should include linux/tracepoint.h.
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In include/trace/events/subsys.h :
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#undef TRACE_SYSTEM
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#define TRACE_SYSTEM subsys
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#if !defined(_TRACE_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
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#define _TRACE_SUBSYS_H
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#include <linux/tracepoint.h>
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DECLARE_TRACE(subsys_eventname,
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TP_PROTO(int firstarg, struct task_struct *p),
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TP_ARGS(firstarg, p));
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#endif /* _TRACE_SUBSYS_H */
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/* This part must be outside protection */
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#include <trace/define_trace.h>
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In subsys/file.c (where the tracing statement must be added) :
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#include <trace/events/subsys.h>
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#define CREATE_TRACE_POINTS
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DEFINE_TRACE(subsys_eventname);
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void somefct(void)
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{
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...
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trace_subsys_eventname(arg, task);
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...
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}
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Where :
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- subsys_eventname is an identifier unique to your event
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- subsys is the name of your subsystem.
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- eventname is the name of the event to trace.
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- TP_PROTO(int firstarg, struct task_struct *p) is the prototype of the
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function called by this tracepoint.
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- TP_ARGS(firstarg, p) are the parameters names, same as found in the
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prototype.
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- if you use the header in multiple source files, #define CREATE_TRACE_POINTS
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should appear only in one source file.
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Connecting a function (probe) to a tracepoint is done by providing a
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probe (function to call) for the specific tracepoint through
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register_trace_subsys_eventname(). Removing a probe is done through
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unregister_trace_subsys_eventname(); it will remove the probe.
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tracepoint_synchronize_unregister() must be called before the end of
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the module exit function to make sure there is no caller left using
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the probe. This, and the fact that preemption is disabled around the
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probe call, make sure that probe removal and module unload are safe.
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The tracepoint mechanism supports inserting multiple instances of the
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same tracepoint, but a single definition must be made of a given
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tracepoint name over all the kernel to make sure no type conflict will
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occur. Name mangling of the tracepoints is done using the prototypes
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to make sure typing is correct. Verification of probe type correctness
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is done at the registration site by the compiler. Tracepoints can be
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put in inline functions, inlined static functions, and unrolled loops
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as well as regular functions.
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The naming scheme "subsys_event" is suggested here as a convention
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intended to limit collisions. Tracepoint names are global to the
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kernel: they are considered as being the same whether they are in the
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core kernel image or in modules.
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If the tracepoint has to be used in kernel modules, an
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EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be
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used to export the defined tracepoints.
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If you need to do a bit of work for a tracepoint parameter, and
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that work is only used for the tracepoint, that work can be encapsulated
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within an if statement with the following:
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if (trace_foo_bar_enabled()) {
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int i;
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int tot = 0;
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for (i = 0; i < count; i++)
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tot += calculate_nuggets();
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trace_foo_bar(tot);
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}
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All trace_<tracepoint>() calls have a matching trace_<tracepoint>_enabled()
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function defined that returns true if the tracepoint is enabled and
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false otherwise. The trace_<tracepoint>() should always be within the
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block of the if (trace_<tracepoint>_enabled()) to prevent races between
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the tracepoint being enabled and the check being seen.
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The advantage of using the trace_<tracepoint>_enabled() is that it uses
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the static_key of the tracepoint to allow the if statement to be implemented
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with jump labels and avoid conditional branches.
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Note: The convenience macro TRACE_EVENT provides an alternative way to
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define tracepoints. Check http://lwn.net/Articles/379903,
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http://lwn.net/Articles/381064 and http://lwn.net/Articles/383362
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for a series of articles with more details.
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