Commit 8741db53 authored by Steven Rostedt's avatar Steven Rostedt Committed by Steven Rostedt

tracing/fgraph: Add max_graph_depth to limit function_graph depth

Add the file max_graph_depth to the debug tracing directory that lets
the user define the depth of the function graph.

A very useful operation is to set the depth to 1. Then it traces only
the first function that is called when entering the kernel. This can
be used to determine what system operations interrupt a process.

For example, to work on NOHZ processes (single tasks running without
a timer tick), if any interrupt goes off and preempts that task, this
code will show it happening.

  # cd /sys/kernel/debug/tracing
  # echo 1 > max_graph_depth
  # echo function_graph > current_tracer
  # cat per_cpu/cpu/<cpu-of-process>/trace

Cc: Frederic Weisbecker <>
Signed-off-by: default avatarSteven Rostedt <>
parent 0f1ac8fd
......@@ -47,6 +47,8 @@ struct fgraph_data {
static unsigned int max_depth;
static struct tracer_opt trace_opts[] = {
/* Display overruns? (for self-debug purpose) */
......@@ -250,8 +252,9 @@ int trace_graph_entry(struct ftrace_graph_ent *trace)
return 0;
/* trace it when it is-nested-in or is a function enabled. */
if (!(trace->depth || ftrace_graph_addr(trace->func)) ||
if ((!(trace->depth || ftrace_graph_addr(trace->func)) ||
ftrace_graph_ignore_irqs()) ||
(max_depth && trace->depth >= max_depth))
return 0;
......@@ -1457,6 +1460,59 @@ static struct tracer graph_trace __read_mostly = {
static ssize_t
graph_depth_write(struct file *filp, const char __user *ubuf, size_t cnt,
loff_t *ppos)
unsigned long val;
int ret;
ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
if (ret)
return ret;
max_depth = val;
*ppos += cnt;
return cnt;
static ssize_t
graph_depth_read(struct file *filp, char __user *ubuf, size_t cnt,
loff_t *ppos)
char buf[15]; /* More than enough to hold UINT_MAX + "\n"*/
int n;
n = sprintf(buf, "%d\n", max_depth);
return simple_read_from_buffer(ubuf, cnt, ppos, buf, n);
static const struct file_operations graph_depth_fops = {
.open = tracing_open_generic,
.write = graph_depth_write,
.read = graph_depth_read,
.llseek = generic_file_llseek,
static __init int init_graph_debugfs(void)
struct dentry *d_tracer;
d_tracer = tracing_init_dentry();
if (!d_tracer)
return 0;
trace_create_file("max_graph_depth", 0644, d_tracer,
NULL, &graph_depth_fops);
return 0;
static __init int init_graph_trace(void)
max_bytes_for_cpu = snprintf(NULL, 0, "%d", nr_cpu_ids - 1);
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment