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>
The sample missed the moving of the header files into the events subdirectory.
I've also extended it based on the existing headers, and mentioned the tiny
but important role of CREATE_TRACE_POINTS.
Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
