path: root/include/linux/tracepoint.h
diff options
authorSteven Rostedt <rostedt@goodmis.org>2014-07-25 16:05:29 -0400
committerSteven Rostedt <rostedt@goodmis.org>2014-08-07 20:39:36 -0400
commit3c49b52b155d0f723792377e1a4480a0e7ca0ba2 (patch)
tree651052ab8141bb7b0a28d23ea02624727632f311 /include/linux/tracepoint.h
parent19583ca584d6f574384e17fe7613dfaeadcdc4a6 (diff)
tracing: Do not do anything special with tracepoint_string when tracing is disabled
When CONFIG_TRACING is not enabled, there's no reason to save the trace strings either by the linker or as a static variable that can be referenced later. Simply pass back the string that is given to tracepoint_string(). Had to move the define to include/linux/tracepoint.h so that it is still visible when CONFIG_TRACING is not set. Link: http://lkml.kernel.org/p/1406318733-26754-2-git-send-email-nicolas.pitre@linaro.org Suggested-by: Nicolas Pitre <nico@linaro.org> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Diffstat (limited to 'include/linux/tracepoint.h')
1 files changed, 44 insertions, 0 deletions
diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 2e2a5f7717e5..b1293f15f592 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -249,6 +249,50 @@ extern void syscall_unregfunc(void);
+ * tracepoint_string - register constant persistent string to trace system
+ * @str - a constant persistent string that will be referenced in tracepoints
+ *
+ * If constant strings are being used in tracepoints, it is faster and
+ * more efficient to just save the pointer to the string and reference
+ * that with a printf "%s" instead of saving the string in the ring buffer
+ * and wasting space and time.
+ *
+ * The problem with the above approach is that userspace tools that read
+ * the binary output of the trace buffers do not have access to the string.
+ * Instead they just show the address of the string which is not very
+ * useful to users.
+ *
+ * With tracepoint_string(), the string will be registered to the tracing
+ * system and exported to userspace via the debugfs/tracing/printk_formats
+ * file that maps the string address to the string text. This way userspace
+ * tools that read the binary buffers have a way to map the pointers to
+ * the ASCII strings they represent.
+ *
+ * The @str used must be a constant string and persistent as it would not
+ * make sense to show a string that no longer exists. But it is still fine
+ * to be used with modules, because when modules are unloaded, if they
+ * had tracepoints, the ring buffers are cleared too. As long as the string
+ * does not change during the life of the module, it is fine to use
+ * tracepoint_string() within a module.
+ */
+#define tracepoint_string(str) \
+ ({ \
+ static const char *___tp_str __tracepoint_string = str; \
+ ___tp_str; \
+ })
+#define __tracepoint_string __attribute__((section("__tracepoint_str")))
+ * tracepoint_string() is used to save the string address for userspace
+ * tracing tools. When tracing isn't configured, there's no need to save
+ * anything.
+ */
+# define tracepoint_string(str) str
+# define __tracepoint_string
* The need for the DECLARE_TRACE_NOARGS() is to handle the prototype
* (void). "void" is a special value in a function prototype and can