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authorSteven Rostedt (Red Hat) <rostedt@goodmis.org>2015-02-09 16:05:55 -0500
committerSteven Rostedt <rostedt@goodmis.org>2015-02-09 16:05:55 -0500
commitc4c7eb29382c456b9be9858c357a490ae0ccd0f6 (patch)
tree96116b672ee0fa5a2347828b33c31fb8dbed6434 /samples
parent4e20e3a60b57efa1e5c26324ce0260d58be6c81b (diff)
downloadkernel-c4c7eb29382c456b9be9858c357a490ae0ccd0f6.tar.gz
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>
Diffstat (limited to 'samples')
-rw-r--r--samples/trace_events/trace-events-sample.c3
-rw-r--r--samples/trace_events/trace-events-sample.h58
2 files changed, 61 insertions, 0 deletions
diff --git a/samples/trace_events/trace-events-sample.c b/samples/trace_events/trace-events-sample.c
index 16c15c08ed38..c396a49b5d78 100644
--- a/samples/trace_events/trace-events-sample.c
+++ b/samples/trace_events/trace-events-sample.c
@@ -31,8 +31,11 @@ static void simple_thread_func(int cnt)
array[i] = i + 1;
array[i] = 0;
+ /* Silly tracepoints */
trace_foo_bar("hello", cnt, array, random_strings[len],
tsk_cpus_allowed(current));
+
+ trace_foo_bar_with_cond("Some times print", cnt);
}
static int simple_thread(void *arg)
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. *****/