tracing: keep ring buffer to minimum size till used

Impact: less memory impact on systems not using tracer

When the kernel boots up that has tracing configured, it allocates
the default size of the ring buffer. This currently happens to be
1.4Megs per possible CPU. This is quite a bit of wasted memory if
the system is never using the tracer.

The current solution is to keep the ring buffers to a minimum size
until the user uses them. Once a tracer is piped into the current_tracer
the ring buffer will be expanded to the default size. If the user
changes the size of the ring buffer, it will take the size given
by the user immediately.

If the user adds a "ftrace=" to the kernel command line, then the ring
buffers will be set to the default size on initialization.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>

kernel/trace/trace.c

@@ -44,6 +44,12 @@
 unsigned long __read_mostly	tracing_max_latency;
 unsigned long __read_mostly	tracing_thresh;
 
+/*
+ * On boot up, the ring buffer is set to the minimum size, so that
+ * we do not waste memory on systems that are not using tracing.
+ */
+static int ring_buffer_expanded;
+
 /*
  * We need to change this state when a selftest is running.
  * A selftest will lurk into the ring-buffer to count the
@@ -128,6 +134,8 @@ static int __init set_ftrace(char *str)
 {
 	strncpy(bootup_tracer_buf, str, BOOTUP_TRACER_SIZE);
 	default_bootup_tracer = bootup_tracer_buf;
+	/* We are using ftrace early, expand it */
+	ring_buffer_expanded = 1;
 	return 1;
 }
 __setup("ftrace=", set_ftrace);
@@ -2315,6 +2323,40 @@ int tracer_init(struct tracer *t, struct trace_array *tr)
 	return t->init(tr);
 }
 
+static int tracing_resize_ring_buffer(unsigned long size)
+{
+	int ret;
+
+	/*
+	 * If kernel or user changes the size of the ring buffer
+	 * it get completed.
+	 */
+	ring_buffer_expanded = 1;
+
+	ret = ring_buffer_resize(global_trace.buffer, size);
+	if (ret < 0)
+		return ret;
+
+	ret = ring_buffer_resize(max_tr.buffer, size);
+	if (ret < 0) {
+		int r;
+
+		r = ring_buffer_resize(global_trace.buffer,
+				       global_trace.entries);
+		if (r < 0) {
+			/* AARGH! We are left with different
+			 * size max buffer!!!! */
+			WARN_ON(1);
+			tracing_disabled = 1;
+		}
+		return ret;
+	}
+
+	global_trace.entries = size;
+
+	return ret;
+}
+
 struct trace_option_dentry;
 
 static struct trace_option_dentry *
@@ -2330,6 +2372,13 @@ static int tracing_set_tracer(const char *buf)
 	struct tracer *t;
 	int ret = 0;
 
+	if (!ring_buffer_expanded) {
+		ret = tracing_resize_ring_buffer(trace_buf_size);
+		if (ret < 0)
+			return ret;
+		ret = 0;
+	}
+
 	mutex_lock(&trace_types_lock);
 	for (t = trace_types; t; t = t->next) {
 		if (strcmp(t->name, buf) == 0)
@@ -2903,28 +2952,11 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
 	val <<= 10;
 
 	if (val != global_trace.entries) {
-		ret = ring_buffer_resize(global_trace.buffer, val);
+		ret = tracing_resize_ring_buffer(val);
 		if (ret < 0) {
 			cnt = ret;
 			goto out;
 		}
-
-		ret = ring_buffer_resize(max_tr.buffer, val);
-		if (ret < 0) {
-			int r;
-			cnt = ret;
-			r = ring_buffer_resize(global_trace.buffer,
-					       global_trace.entries);
-			if (r < 0) {
-				/* AARGH! We are left with different
-				 * size max buffer!!!! */
-				WARN_ON(1);
-				tracing_disabled = 1;
-			}
-			goto out;
-		}
-
-		global_trace.entries = val;
 	}
 
 	filp->f_pos += cnt;
@@ -3916,6 +3948,7 @@ void ftrace_dump(void)
 __init static int tracer_alloc_buffers(void)
 {
 	struct trace_array_cpu *data;
+	int ring_buf_size;
 	int i;
 	int ret = -ENOMEM;
 
@@ -3928,12 +3961,18 @@ __init static int tracer_alloc_buffers(void)
 	if (!alloc_cpumask_var(&tracing_reader_cpumask, GFP_KERNEL))
 		goto out_free_tracing_cpumask;
 
+	/* To save memory, keep the ring buffer size to its minimum */
+	if (ring_buffer_expanded)
+		ring_buf_size = trace_buf_size;
+	else
+		ring_buf_size = 1;
+
 	cpumask_copy(tracing_buffer_mask, cpu_possible_mask);
 	cpumask_copy(tracing_cpumask, cpu_all_mask);
 	cpumask_clear(tracing_reader_cpumask);
 
 	/* TODO: make the number of buffers hot pluggable with CPUS */
-	global_trace.buffer = ring_buffer_alloc(trace_buf_size,
+	global_trace.buffer = ring_buffer_alloc(ring_buf_size,
 						   TRACE_BUFFER_FLAGS);
 	if (!global_trace.buffer) {
 		printk(KERN_ERR "tracer: failed to allocate ring buffer!\n");
@@ -3944,7 +3983,7 @@ __init static int tracer_alloc_buffers(void)
 
 
 #ifdef CONFIG_TRACER_MAX_TRACE
-	max_tr.buffer = ring_buffer_alloc(trace_buf_size,
+	max_tr.buffer = ring_buffer_alloc(ring_buf_size,
 					     TRACE_BUFFER_FLAGS);
 	if (!max_tr.buffer) {
 		printk(KERN_ERR "tracer: failed to allocate max ring buffer!\n");