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>

5 files changed, 89 insertions, 0 deletions

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 16e5bb5daaa5..d7b2de744631 100644
--- a/kernel/trace/Kconfig
+++ b/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
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index 58ec61c44bd6..73ba13f5a461 100644
--- a/kernel/trace/Makefile
+++ b/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
 
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 42f65d0097f0..447d4b9b6391 100644
--- a/kernel/trace/trace.h
+++ b/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);
diff --git a/kernel/trace/trace_nop.c b/kernel/trace/trace_nop.c
new file mode 100644
index 000000000000..dafaefb84038
--- /dev/null
+++ b/kernel/trace/trace_nop.c
@@ -0,0 +1,65 @@
+/*
+ * 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);
diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
index 630715bbd572..82db9103b9bc 100644
--- a/kernel/trace/trace_selftest.c
+++ b/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)
 {