tracing: Add trace_trigger kernel command line option

Allow triggers to be enabled at kernel boot up. For example:

trace_trigger="sched_switch.stacktrace if prev_state == 2"

The above will enable the stacktrace trigger on top of the sched_switch
event and only trigger if its prev_state is 2 (TASK_UNINTERRUPTIBLE). Then
at boot up, a stacktrace will trigger and be recorded in the tracing ring
buffer every time the sched_switch happens where the previous state is
TASK_INTERRUPTIBLE.

Another useful trigger would be "traceoff" which can stop tracing on an
event if a field of the event matches a certain value defined by the
filter ("if" statement).

Link: https://lore.kernel.org/linux-trace-kernel/[e-mail]

Signed-off-by: Steven Rostedt (Google) <[e-mail]>

Documentation/admin-guide/kernel-parameters.txt

···
 			See also Documentation/trace/ftrace.rst "trace options"
 			section.
 
+	trace_trigger=[trigger-list]
+			[FTRACE] Add a event trigger on specific events.
+			Set a trigger on top of a specific event, with an optional
+			filter.
+
+			The format is is "trace_trigger=<event>.<trigger>[ if <filter>],..."
+			Where more than one trigger may be specified that are comma deliminated.
+
+			For example:
+
+			  trace_trigger="sched_switch.stacktrace if prev_state == 2"
+
+			The above will enable the "stacktrace" trigger on the "sched_switch"
+			event but only trigger it if the "prev_state" of the "sched_switch"
+			event is "2" (TASK_UNINTERUPTIBLE).
+
+			See also "Event triggers" in Documentation/trace/events.rst
+
+
 	traceoff_on_warning
 			[FTRACE] enable this option to disable tracing when a
 			warning is hit. This turns off "tracing_on". Tracing can

kernel/trace/trace_events.c

···
 	return file;
 }
 
+#ifdef CONFIG_HIST_TRIGGERS
+#define MAX_BOOT_TRIGGERS 32
+
+static struct boot_triggers {
+	const char		*event;
+	char			*trigger;
+} bootup_triggers[MAX_BOOT_TRIGGERS];
+
+static char bootup_trigger_buf[COMMAND_LINE_SIZE];
+static int nr_boot_triggers;
+
+static __init int setup_trace_triggers(char *str)
+{
+	char *trigger;
+	char *buf;
+	int i;
+
+	strlcpy(bootup_trigger_buf, str, COMMAND_LINE_SIZE);
+	ring_buffer_expanded = true;
+	disable_tracing_selftest("running event triggers");
+
+	buf = bootup_trigger_buf;
+	for (i = 0; i < MAX_BOOT_TRIGGERS; i++) {
+		trigger = strsep(&buf, ",");
+		if (!trigger)
+			break;
+		bootup_triggers[i].event = strsep(&trigger, ".");
+		bootup_triggers[i].trigger = strsep(&trigger, ".");
+		if (!bootup_triggers[i].trigger)
+			break;
+	}
+
+	nr_boot_triggers = i;
+	return 1;
+}
+__setup("trace_trigger=", setup_trace_triggers);
+#endif
+
 /* Add an event to a trace directory */
 static int
 __trace_add_new_event(struct trace_event_call *call, struct trace_array *tr)
···
 		return event_define_fields(call);
 }
 
+#ifdef CONFIG_HIST_TRIGGERS
+static void trace_early_triggers(struct trace_event_file *file, const char *name)
+{
+	int ret;
+	int i;
+
+	for (i = 0; i < nr_boot_triggers; i++) {
+		if (strcmp(name, bootup_triggers[i].event))
+			continue;
+		mutex_lock(&event_mutex);
+		ret = trigger_process_regex(file, bootup_triggers[i].trigger);
+		mutex_unlock(&event_mutex);
+		if (ret)
+			pr_err("Failed to register trigger '%s' on event %s\n",
+			       bootup_triggers[i].trigger,
+			       bootup_triggers[i].event);
+	}
+}
+#else
+static inline void trace_early_triggers(struct trace_event_file *file, const char *name) { }
+#endif
+
 /*
  * Just create a descriptor for early init. A descriptor is required
  * for enabling events at boot. We want to enable events before
···
 			    struct trace_array *tr)
 {
 	struct trace_event_file *file;
+	int ret;
 
 	file = trace_create_new_event(call, tr);
 	if (!file)
 		return -ENOMEM;
 
-	return event_define_fields(call);
+	ret = event_define_fields(call);
+	if (ret)
+		return ret;
+
+	trace_early_triggers(file, trace_event_name(call));
+
+	return 0;
 }
 
 struct ftrace_module_file_ops;
···
 			list_add(&call->list, &ftrace_events);
 	}
 
+	register_trigger_cmds();
+
 	/*
 	 * We need the top trace array to have a working set of trace
 	 * points at early init, before the debug files and directories
···
 
 	register_event_cmds();
 
-	register_trigger_cmds();
 
 	return 0;
 }