----- Original Message ----- > From: "Steven Rostedt" <[email protected]> > To: "LKML" <[email protected]> > Cc: "Mathieu Desnoyers" <[email protected]>, "Andrew Morton" > <[email protected]>, "Javi Merino" > <[email protected]>, "David Howells" <[email protected]>, "Ingo Molnar" > <[email protected]> > Sent: Tuesday, May 6, 2014 11:10:59 PM > Subject: [RFA][PATCH v2] tracing: Add trace_<tracepoint>_enabled() function > > > 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/[email protected] > > Signed-off-by: Steven Rostedt <[email protected]>
Acked-by: Mathieu Desnoyers <[email protected]> > --- > Changes since v1: Added some documentation about using this function. > --- > 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_<tracepoint>() calls have a matching trace_<tracepoint>_enabled() > +function defined that returns true if the tracepoint is enabled and > +false otherwise. The trace_<tracepoint>() should always be within the > +block of the if (trace_<tracepoint>_enabled()) to prevent races between > +the tracepoint being enabled and the check being seen. > + > +The advantage of using the trace_<tracepoint>_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) > -- > 1.8.1.4 > > > -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
