ftrace: inject markers via trace_marker file

Allow a user to inject a marker (TRACE_PRINT entry) into the trace ring
buffer. The related file operations are derived from code by Frédéric
Weisbecker <fweisbec@gmail.com>.
Signed-off-by: NPekka Paalanen <pq@iki.fi>
Acked-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Documentation/tracers/mmiotrace.txt

@@ -36,7 +36,7 @@ $ mount -t debugfs debugfs /debug
 $ echo mmiotrace > /debug/tracing/current_tracer
 $ cat /debug/tracing/trace_pipe > mydump.txt &
 Start X or whatever.
-$ echo "X is up" > /debug/tracing/marker
+$ echo "X is up" > /debug/tracing/trace_marker
 $ echo none > /debug/tracing/current_tracer
 Check for lost events.
 
@@ -59,9 +59,8 @@ The 'cat' process should stay running (sleeping) in the background.
 Load the driver you want to trace and use it. Mmiotrace will only catch MMIO
 accesses to areas that are ioremapped while mmiotrace is active.
 
-[Unimplemented feature:]
 During tracing you can place comments (markers) into the trace by
-$ echo "X is up" > /debug/tracing/marker
+$ echo "X is up" > /debug/tracing/trace_marker
 This makes it easier to see which part of the (huge) trace corresponds to
 which action. It is recommended to place descriptive markers about what you
 do.

kernel/trace/trace.c

@@ -2879,6 +2879,66 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
 	return cnt;
 }
 
+static int tracing_open_mark(struct inode *inode, struct file *filp)
+{
+	int ret;
+
+	ret = tracing_open_generic(inode, filp);
+	if (ret)
+		return ret;
+
+	if (current_trace == &no_tracer)
+		return -ENODEV;
+
+	return 0;
+}
+
+static int mark_printk(const char *fmt, ...)
+{
+	int ret;
+	va_list args;
+	va_start(args, fmt);
+	ret = trace_vprintk(0, fmt, args);
+	va_end(args);
+	return ret;
+}
+
+static ssize_t
+tracing_mark_write(struct file *filp, const char __user *ubuf,
+					size_t cnt, loff_t *fpos)
+{
+	char *buf;
+	char *end;
+	struct trace_array *tr = &global_trace;
+
+	if (current_trace == &no_tracer || !tr->ctrl || tracing_disabled)
+		return -EINVAL;
+
+	if (cnt > TRACE_BUF_SIZE)
+		cnt = TRACE_BUF_SIZE;
+
+	buf = kmalloc(cnt + 1, GFP_KERNEL);
+	if (buf == NULL)
+		return -ENOMEM;
+
+	if (copy_from_user(buf, ubuf, cnt)) {
+		kfree(buf);
+		return -EFAULT;
+	}
+
+	/* Cut from the first nil or newline. */
+	buf[cnt] = '\0';
+	end = strchr(buf, '\n');
+	if (end)
+		*end = '\0';
+
+	cnt = mark_printk("%s\n", buf);
+	kfree(buf);
+	*fpos += cnt;
+
+	return cnt;
+}
+
 static struct file_operations tracing_max_lat_fops = {
 	.open		= tracing_open_generic,
 	.read		= tracing_max_lat_read,
@@ -2910,6 +2970,11 @@ static struct file_operations tracing_entries_fops = {
 	.write		= tracing_entries_write,
 };
 
+static struct file_operations tracing_mark_fops = {
+	.open		= tracing_open_mark,
+	.write		= tracing_mark_write,
+};
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 
 static ssize_t
@@ -3027,6 +3092,12 @@ static __init void tracer_init_debugfs(void)
 		pr_warning("Could not create debugfs "
 			   "'trace_entries' entry\n");
 
+	entry = debugfs_create_file("trace_marker", 0220, d_tracer,
+				    NULL, &tracing_mark_fops);
+	if (!entry)
+		pr_warning("Could not create debugfs "
+			   "'trace_marker' entry\n");
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 	entry = debugfs_create_file("dyn_ftrace_total_info", 0444, d_tracer,
 				    &ftrace_update_tot_cnt,
@@ -3040,11 +3111,6 @@ static __init void tracer_init_debugfs(void)
 #endif
 }
 
-#define TRACE_BUF_SIZE 1024
-#define TRACE_PRINT_BUF_SIZE \
-	(sizeof(struct trace_field) - offsetof(struct trace_field, print.buf))
-#define TRACE_CONT_BUF_SIZE sizeof(struct trace_field)
-
 int trace_vprintk(unsigned long ip, const char *fmt, va_list args)
 {
 	static DEFINE_SPINLOCK(trace_buf_lock);

kernel/trace/trace.h

@@ -124,6 +124,10 @@ struct trace_entry {
 };
 
 #define TRACE_ENTRY_SIZE	sizeof(struct trace_entry)
+#define TRACE_BUF_SIZE		1024
+#define TRACE_PRINT_BUF_SIZE \
+	(sizeof(struct trace_field) - offsetof(struct trace_field, print.buf))
+#define TRACE_CONT_BUF_SIZE	sizeof(struct trace_field)
 
 /*
  * The CPU trace array - it consists of thousands of trace entries