tracing: Add TRACE_EVENT_CONDITION sample

The sample code lacks an example of TRACE_EVENT_CONDITION, and it
has been expressed to me that this feature for TRACE_EVENT is not
well known and not used when it could be.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 58 insertions, 0 deletions

diff --git a/samples/trace_events/trace-events-sample.h b/samples/trace_events/trace-events-sample.h
index dd65f7b8c0d9..c3232340914d 100644
--- a/samples/trace_events/trace-events-sample.h
+++ b/samples/trace_events/trace-events-sample.h
@@ -212,6 +212,64 @@ TRACE_EVENT(foo_bar,
                                 sizeof(int)),
                   __get_str(str), __get_bitmask(cpus))
 );
+
+/*
+ * There may be a case where a tracepoint should only be called if
+ * some condition is set. Otherwise the tracepoint should not be called.
+ * But to do something like:
+ *
+ *  if (cond)
+ *     trace_foo();
+ *
+ * Would cause a little overhead when tracing is not enabled, and that
+ * overhead, even if small, is not something we want. As tracepoints
+ * use static branch (aka jump_labels), where no branch is taken to
+ * skip the tracepoint when not enabled, and a jmp is placed to jump
+ * to the tracepoint code when it is enabled, having a if statement
+ * nullifies that optimization. It would be nice to place that
+ * condition within the static branch. This is where TRACE_EVENT_CONDITION
+ * comes in.
+ *
+ * TRACE_EVENT_CONDITION() is just like TRACE_EVENT, except it adds another
+ * parameter just after args. Where TRACE_EVENT has:
+ *
+ * TRACE_EVENT(name, proto, args, struct, assign, printk)
+ *
+ * the CONDITION version has:
+ *
+ * TRACE_EVENT_CONDITION(name, proto, args, cond, struct, assign, printk)
+ *
+ * Everything is the same as TRACE_EVENT except for the new cond. Think
+ * of the cond variable as:
+ *
+ *   if (cond)
+ *      trace_foo_bar_with_cond();
+ *
+ * Except that the logic for the if branch is placed after the static branch.
+ * That is, the if statement that processes the condition will not be
+ * executed unless that traecpoint is enabled. Otherwise it still remains
+ * a nop.
+ */
+TRACE_EVENT_CONDITION(foo_bar_with_cond,
+
+        TP_PROTO(const char *foo, int bar),
+
+        TP_ARGS(foo, bar),
+
+        TP_CONDITION(!(bar % 10)),
+
+        TP_STRUCT__entry(
+                __string(       foo,    foo             )
+                __field(        int,    bar                     )
+        ),
+
+        TP_fast_assign(
+                __assign_str(foo, foo);
+                __entry->bar    = bar;
+        ),
+
+        TP_printk("foo %s %d", __get_str(foo), __entry->bar)
+);
 #endif
 
 /***** NOTICE! The #if protection ends here. *****/