tracing: Add __tracepoint_string() to export string pointers

[~andy/linux] / include / linux / ftrace_event.h

diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
index 4372658c73ae5eddd3dd6570d4dfac8c5d8825c8..81af18a75f4db365cd4e57523060f13e33104481 100644 (file)
--- a/include/linux/ftrace_event.h
+++ b/include/linux/ftrace_event.h
@@ -357,6 +357,40 @@ do {                                                                       \
                __trace_printk(ip, fmt, ##args);                        \
 } while (0)
 
+/**
+ * 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")))
+
 #ifdef CONFIG_PERF_EVENTS
 struct perf_event;