trace: let boot trace be chosen by command line

Now that we have a working ftrace=<tracer> function, make the boot
tracer get activated by it. This way we can turn it on or off without
recompiling the kernel, as well as keeping the selftests on. The
selftests are disabled whenever a default tracer starts running.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/Kconfig

@@ -164,9 +164,8 @@ config BOOT_TRACER
 	  representation of the delays during initcalls - but the raw
 	  /debug/tracing/trace text output is readable too.
 
-	  ( Note that tracing self tests can't be enabled if this tracer is
-	    selected, because the self-tests are an initcall as well and that
-	    would invalidate the boot trace. )
+	  You must pass in ftrace=initcall to the kernel command line
+	  to enable this on bootup.
 
 config TRACE_BRANCH_PROFILING
 	bool "Trace likely/unlikely profiler"
@@ -326,7 +325,7 @@ config FTRACE_SELFTEST
 
 config FTRACE_STARTUP_TEST
 	bool "Perform a startup test on ftrace"
-	depends on TRACING && DEBUG_KERNEL && !BOOT_TRACER
+	depends on TRACING && DEBUG_KERNEL
 	select FTRACE_SELFTEST
 	help
 	  This option performs a series of startup tests on ftrace. On bootup

kernel/trace/trace.c

@@ -3167,12 +3167,9 @@ __init static int tracer_alloc_buffers(void)
 	trace_init_cmdlines();
 
 	register_tracer(&nop_trace);
+	current_trace = &nop_trace;
 #ifdef CONFIG_BOOT_TRACER
 	register_tracer(&boot_tracer);
-	current_trace = &boot_tracer;
-	current_trace->init(&global_trace);
-#else
-	current_trace = &nop_trace;
 #endif
 	/* All seems OK, enable tracing */
 	tracing_disabled = 0;

kernel/trace/trace_boot.c

@@ -28,13 +28,13 @@ void start_boot_trace(void)
 
 void enable_boot_trace(void)
 {
-	if (pre_initcalls_finished)
+	if (boot_trace && pre_initcalls_finished)
 		tracing_start_sched_switch_record();
 }
 
 void disable_boot_trace(void)
 {
-	if (pre_initcalls_finished)
+	if (boot_trace && pre_initcalls_finished)
 		tracing_stop_sched_switch_record();
 }
 
@@ -43,6 +43,9 @@ static int boot_trace_init(struct trace_array *tr)
 	int cpu;
 	boot_trace = tr;
 
+	if (!tr)
+		return 0;
+
 	for_each_cpu(cpu, cpu_possible_mask)
 		tracing_reset(tr, cpu);
 
@@ -132,7 +135,7 @@ void trace_boot_call(struct boot_trace_call *bt, initcall_t fn)
 	unsigned long irq_flags;
 	struct trace_array *tr = boot_trace;
 
-	if (!pre_initcalls_finished)
+	if (!tr || !pre_initcalls_finished)
 		return;
 
 	/* Get its name now since this function could
@@ -164,7 +167,7 @@ void trace_boot_ret(struct boot_trace_ret *bt, initcall_t fn)
 	unsigned long irq_flags;
 	struct trace_array *tr = boot_trace;
 
-	if (!pre_initcalls_finished)
+	if (!tr || !pre_initcalls_finished)
 		return;
 
 	sprint_symbol(bt->func, (unsigned long)fn);