From: "Steven Rostedt (Red Hat)" 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__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 Signed-off-by: Steven Rostedt --- Documentation/trace/tracepoints.txt | 24 ++++++++++++++++++++++++ include/linux/tracepoint.h | 10 ++++++++++ 2 files changed, 34 insertions(+) diff --git a/Documentation/trace/tracepoints.txt b/Documentation/trace/tracepoints.txt index 6b018b5..a3efac6 100644 --- a/Documentation/trace/tracepoints.txt +++ b/Documentation/trace/tracepoints.txt @@ -115,6 +115,30 @@ If the tracepoint has to be used in kernel modules, an EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be used to export the defined tracepoints. +If you need to do a bit of work for a tracepoint parameter, and +that work is only used for the tracepoint, that work can be encapsulated +within an if statement with the following: + + if (trace_foo_bar_enabled()) { + int i; + int tot = 0; + + for (i = 0; i < count; i++) + tot += calculate_nuggets(); + + trace_foo_bar(tot); + } + +All trace_() calls have a matching trace__enabled() +function defined that returns true if the tracepoint is enabled and +false otherwise. The trace_() should always be within the +block of the if (trace__enabled()) to prevent races between +the tracepoint being enabled and the check being seen. + +The advantage of using the trace__enabled() is that it uses +the static_key of the tracepoint to allow the if statement to be implemented +with jump labels and avoid conditional branches. + Note: The convenience macro TRACE_EVENT provides an alternative way to define tracepoints. Check http://lwn.net/Articles/379903, http://lwn.net/Articles/381064 and http://lwn.net/Articles/383362 diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h index 9d30ee4..2e2a5f7 100644 --- a/include/linux/tracepoint.h +++ b/include/linux/tracepoint.h @@ -185,6 +185,11 @@ extern void syscall_unregfunc(void); static inline void \ check_trace_callback_type_##name(void (*cb)(data_proto)) \ { \ + } \ + static inline bool \ + trace_##name##_enabled(void) \ + { \ + return static_key_false(&__tracepoint_##name.key); \ } /* @@ -230,6 +235,11 @@ extern void syscall_unregfunc(void); } \ static inline void check_trace_callback_type_##name(void (*cb)(data_proto)) \ { \ + } \ + static inline bool \ + trace_##name##_enabled(void) \ + { \ + return false; \ } #define DEFINE_TRACE_FN(name, reg, unreg) -- 1.8.5.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/