ftrace: add nop tracer

A no-op tracer which can serve two purposes:

 1. A template for development of a new tracer.
 2. A convenient way to see ftrace_printk() calls without
    an irrelevant trace making the output messy.

[ mingo@elte.hu: resolved conflicts ]
Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/Kconfig

@@ -101,6 +101,16 @@ config SCHED_TRACER
 	  This tracer tracks the latency of the highest priority task
 	  to be scheduled in, starting from the point it has woken up.
 
+config NOP_TRACER
+	bool "NOP Tracer"
+	depends on HAVE_FTRACE
+	depends on DEBUG_KERNEL
+	select TRACING
+	help
+	  This tracer does nothing. The primary purpose for it is to
+	  politely print the output of ftrace_printk() calls without
+	  the overhead of an irrelevant trace taking place.
+
 config CONTEXT_SWITCH_TRACER
 	bool "Trace process context switches"
 	depends on HAVE_FTRACE

kernel/trace/Makefile

@@ -19,6 +19,7 @@ obj-$(CONFIG_FTRACE) += trace_functions.o
 obj-$(CONFIG_IRQSOFF_TRACER) += trace_irqsoff.o
 obj-$(CONFIG_PREEMPT_TRACER) += trace_irqsoff.o
 obj-$(CONFIG_SCHED_TRACER) += trace_sched_wakeup.o
+obj-$(CONFIG_NOP_TRACER) += trace_nop.o
 obj-$(CONFIG_STACK_TRACER) += trace_stack.o
 obj-$(CONFIG_MMIOTRACE) += trace_mmiotrace.o
 

kernel/trace/trace.h

@@ -339,6 +339,10 @@ extern int trace_selftest_startup_preemptirqsoff(struct tracer *trace,
 extern int trace_selftest_startup_wakeup(struct tracer *trace,
 					 struct trace_array *tr);
 #endif
+#ifdef CONFIG_NOP_TRACER
+extern int trace_selftest_startup_nop(struct tracer *trace,
+					 struct trace_array *tr);
+#endif
 #ifdef CONFIG_CONTEXT_SWITCH_TRACER
 extern int trace_selftest_startup_sched_switch(struct tracer *trace,
 					       struct trace_array *tr);

kernel/trace/trace_nop.c

+/*
+ * nop tracer
+ *
+ * Copyright (C) 2008 Steven Noonan <steven@uplinklabs.net>
+ *
+ */
+
+#include <linux/module.h>
+#include <linux/fs.h>
+#include <linux/debugfs.h>
+#include <linux/ftrace.h>
+
+#include "trace.h"
+
+static struct trace_array	*ctx_trace;
+
+static void start_nop_trace(struct trace_array *tr)
+{
+	/* Nothing to do! */
+}
+
+static void stop_nop_trace(struct trace_array *tr)
+{
+	/* Nothing to do! */
+}
+
+static void nop_trace_init(struct trace_array *tr)
+{
+	ctx_trace = tr;
+
+	if (tr->ctrl)
+		start_nop_trace(tr);
+}
+
+static void nop_trace_reset(struct trace_array *tr)
+{
+	if (tr->ctrl)
+		stop_nop_trace(tr);
+}
+
+static void nop_trace_ctrl_update(struct trace_array *tr)
+{
+	/* When starting a new trace, reset the buffers */
+	if (tr->ctrl)
+		start_nop_trace(tr);
+	else
+		stop_nop_trace(tr);
+}
+
+static struct tracer nop_trace __read_mostly =
+{
+	.name		= "nop",
+	.init		= nop_trace_init,
+	.reset		= nop_trace_reset,
+	.ctrl_update	= nop_trace_ctrl_update,
+#ifdef CONFIG_FTRACE_SELFTEST
+	.selftest	= trace_selftest_startup_nop,
+#endif
+};
+
+__init static int init_nop_trace(void)
+{
+	return register_tracer(&nop_trace);
+}
+device_initcall(init_nop_trace);

kernel/trace/trace_selftest.c

@@ -418,6 +418,15 @@ trace_selftest_startup_preemptirqsoff(struct tracer *trace, struct trace_array *
 }
 #endif /* CONFIG_IRQSOFF_TRACER && CONFIG_PREEMPT_TRACER */
 
+#ifdef CONFIG_NOP_TRACER
+int
+trace_selftest_startup_nop(struct tracer *trace, struct trace_array *tr)
+{
+	/* What could possibly go wrong? */
+	return 0;
+}
+#endif
+
 #ifdef CONFIG_SCHED_TRACER
 static int trace_wakeup_test_thread(void *data)
 {