[PATCH v2] tracing: Allow changing default ring buffer size for ftrace instances.

[Bharath Ravi <rbharath@google.com>]

It is often memory efficient to start instances off with a smaller ring
buffer size than the current default. This is particularly true on
systems with many cores, or when multiple ftrace instances are created,
where the current (higher) default value results in allocation failures.

The global trace buffer allows initialization with a minimal (1 byte)
size, to save memory using a "ring_buffer_expanded" flag. To allow
instances a similar capability, trace option named "expand-new-buffers"
is added. If set to true, ftrace instances created will be initialized
with ring buffers of the default size. If set to false, the buffers will
be initialized with a size of 1 byte. By default, the expand_new_buffers
flag is true.

Tested: Booted into a kernel with these changes, verified reading and
writing the new trace_option. Ensured that new ftrace instances created
used the appropriate size while initializing their ring buffers.

Signed-off-by: Bharath Ravi <rbharath@google.com>
---
 kernel/trace/trace.c | 13 ++++++++++---
 kernel/trace/trace.h |  1 +
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 9d20cd9..c17c12f 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -410,7 +410,8 @@ static inline void trace_access_lock_init(void)
 unsigned long trace_flags = TRACE_ITER_PRINT_PARENT | TRACE_ITER_PRINTK |
 	TRACE_ITER_ANNOTATE | TRACE_ITER_CONTEXT_INFO | TRACE_ITER_SLEEP_TIME |
 	TRACE_ITER_GRAPH_TIME | TRACE_ITER_RECORD_CMD | TRACE_ITER_OVERWRITE |
-	TRACE_ITER_IRQ_INFO | TRACE_ITER_MARKERS | TRACE_ITER_FUNCTION;
+	TRACE_ITER_IRQ_INFO | TRACE_ITER_MARKERS | TRACE_ITER_FUNCTION |
+	TRACE_EXPAND_NEW_BUFFERS;
 
 static void tracer_tracing_on(struct trace_array *tr)
 {
@@ -753,6 +754,7 @@ static const char *trace_options[] = {
 	"irq-info",
 	"markers",
 	"function-trace",
+	"expand-new-buffers",
 	NULL
 };
 
@@ -5930,7 +5932,7 @@ static int allocate_trace_buffers(struct trace_array *tr, int size)
 static int new_instance_create(const char *name)
 {
 	struct trace_array *tr;
-	int ret;
+	int ret, ring_buffer_size;
 
 	mutex_lock(&trace_types_lock);
 
@@ -5961,7 +5963,12 @@ static int new_instance_create(const char *name)
 	INIT_LIST_HEAD(&tr->systems);
 	INIT_LIST_HEAD(&tr->events);
 
-	if (allocate_trace_buffers(tr, trace_buf_size) < 0)
+	/* allocate memory only if buffers are to be expanded */
+	if (trace_flags & TRACE_EXPAND_NEW_BUFFERS)
+		ring_buffer_size = trace_buf_size;
+	else
+		ring_buffer_size = 1
+	if (allocate_trace_buffers(tr, ring_buffer_size) < 0)
 		goto out_free_tr;
 
 	tr->dir = debugfs_create_dir(name, trace_instance_dir);
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index ea189e0..15ddbf4 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -888,6 +888,7 @@ enum trace_iterator_flags {
 	TRACE_ITER_IRQ_INFO		= 0x800000,
 	TRACE_ITER_MARKERS		= 0x1000000,
 	TRACE_ITER_FUNCTION		= 0x2000000,
+	TRACE_EXPAND_NEW_BUFFERS	= 0x4000000,
 };
 
 /*
-- 
1.8.5.3