Re: [PATCHv2] tracing: let user specify tracing_thresh after selecting function_graph

[Stanislav Fomichev <stfomichev@yandex-team.ru>]

> Ideally there should be a callback for the tracer to deal with it, but
> instead, you can for now just hardcode a function graph tracer callback
> into tracing_max_lat_write(). Something like this:
> 
> 	if (ptr == &tracing_thresh)
> 		ftrace_update_func_graph_thresh();
> 
> And then if the function tracer is active, stop it, and then restart it
> with the tracing thresh function.
Attached new patch which adds new callback and uses it in function_graph
tracer (not sure whether I need to check graph_array in
graph_trace_update_thresh). Maybe it makes sense to split it into two commits
(add callback itself, use it in the function_graph tracer)?

Btw, I also tried to use static_keys/jump_labels in places where
tracing_thresh is checked, but got double faults. Is there a reason I
can't use jump labels inside tracers?

---

Currently, tracing_thresh works only if we specify it before selecting
function_graph tracer. If we do the opposite, tracing_thresh will change
it's value, but it will not be applied.
To fix it, we add update_thresh callback which is called whenever
tracing_thresh is updated and for function_graph tracer we register
handler which reinitializes tracer depending on tracing_thresh.

Signed-off-by: Stanislav Fomichev <stfomichev@yandex-team.ru>
---
 kernel/trace/trace.c                 | 60 +++++++++++++++++++++++++++++++-----
 kernel/trace/trace.h                 |  2 ++
 kernel/trace/trace_functions_graph.c | 10 ++++++
 3 files changed, 65 insertions(+), 7 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 737b0efa1a62..06fbb4b7e646 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4077,10 +4077,9 @@ tracing_set_trace_write(struct file *filp, const char __user *ubuf,
 }
 
 static ssize_t
-tracing_max_lat_read(struct file *filp, char __user *ubuf,
-		     size_t cnt, loff_t *ppos)
+nsecs_read(unsigned long *ptr, char __user *ubuf,
+	   size_t cnt, loff_t *ppos)
 {
-	unsigned long *ptr = filp->private_data;
 	char buf[64];
 	int r;
 
@@ -4092,10 +4091,9 @@ tracing_max_lat_read(struct file *filp, char __user *ubuf,
 }
 
 static ssize_t
-tracing_max_lat_write(struct file *filp, const char __user *ubuf,
-		      size_t cnt, loff_t *ppos)
+nsecs_write(unsigned long *ptr, const char __user *ubuf,
+	    size_t cnt, loff_t *ppos)
 {
-	unsigned long *ptr = filp->private_data;
 	unsigned long val;
 	int ret;
 
@@ -4108,6 +4106,47 @@ tracing_max_lat_write(struct file *filp, const char __user *ubuf,
 	return cnt;
 }
 
+static ssize_t
+tracing_thresh_read(struct file *filp, char __user *ubuf,
+		    size_t cnt, loff_t *ppos)
+{
+	return nsecs_read(&tracing_thresh, ubuf, cnt, ppos);
+}
+
+static ssize_t
+tracing_thresh_write(struct file *filp, const char __user *ubuf,
+		     size_t cnt, loff_t *ppos)
+{
+	struct trace_array *tr = filp->private_data;
+	int ret;
+
+	ret = nsecs_write(&tracing_thresh, ubuf, cnt, ppos);
+	if (ret < 0)
+		return ret;
+
+	if (tr->current_trace->update_thresh) {
+		ret = tr->current_trace->update_thresh(tr);
+		if (ret)
+			return ret;
+	}
+
+	return cnt;
+}
+
+static ssize_t
+tracing_max_lat_read(struct file *filp, char __user *ubuf,
+		     size_t cnt, loff_t *ppos)
+{
+	return nsecs_read(filp->private_data, ubuf, cnt, ppos);
+}
+
+static ssize_t
+tracing_max_lat_write(struct file *filp, const char __user *ubuf,
+		      size_t cnt, loff_t *ppos)
+{
+	return nsecs_write(filp->private_data, ubuf, cnt, ppos);
+}
+
 static int tracing_open_pipe(struct inode *inode, struct file *filp)
 {
 	struct trace_array *tr = inode->i_private;
@@ -5024,6 +5063,13 @@ static int snapshot_raw_open(struct inode *inode, struct file *filp)
 #endif /* CONFIG_TRACER_SNAPSHOT */
 
 
+static const struct file_operations tracing_thresh_fops = {
+	.open		= tracing_open_generic,
+	.read		= tracing_thresh_read,
+	.write		= tracing_thresh_write,
+	.llseek		= generic_file_llseek,
+};
+
 static const struct file_operations tracing_max_lat_fops = {
 	.open		= tracing_open_generic,
 	.read		= tracing_max_lat_read,
@@ -6359,7 +6405,7 @@ static __init int tracer_init_debugfs(void)
 #endif
 
 	trace_create_file("tracing_thresh", 0644, d_tracer,
-			&tracing_thresh, &tracing_max_lat_fops);
+			&global_trace, &tracing_thresh_fops);
 
 	trace_create_file("README", 0444, d_tracer,
 			NULL, &tracing_readme_fops);
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 2e29d7ba5a52..960c523de47d 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -321,6 +321,7 @@ struct tracer_flags {
  * @reset: called when one switches to another tracer
  * @start: called when tracing is unpaused (echo 1 > tracing_enabled)
  * @stop: called when tracing is paused (echo 0 > tracing_enabled)
+ * @update_thresh: called when tracing_thresh is updated
  * @open: called when the trace file is opened
  * @pipe_open: called when the trace_pipe file is opened
  * @wait_pipe: override how the user waits for traces on trace_pipe
@@ -340,6 +341,7 @@ struct tracer {
 	void			(*reset)(struct trace_array *tr);
 	void			(*start)(struct trace_array *tr);
 	void			(*stop)(struct trace_array *tr);
+	int			(*update_thresh)(struct trace_array *tr);
 	void			(*open)(struct trace_iterator *iter);
 	void			(*pipe_open)(struct trace_iterator *iter);
 	void			(*wait_pipe)(struct trace_iterator *iter);
diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index deff11200261..7440541a3b4f 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -450,6 +450,15 @@ static void graph_trace_reset(struct trace_array *tr)
 {
 	tracing_stop_cmdline_record();
 	unregister_ftrace_graph();
+	set_graph_array(NULL);
+}
+
+void graph_trace_update_thresh(struct trace_array *tr)
+{
+	if (graph_array) {
+		graph_trace_reset(tr);
+		graph_trace_init(tr);
+	}
 }
 
 static int max_bytes_for_cpu;
@@ -1501,6 +1510,7 @@ static struct trace_event graph_trace_ret_event = {
 
 static struct tracer graph_trace __tracer_data = {
 	.name		= "function_graph",
+	.update_thresh	= graph_trace_update_thresh,
 	.open		= graph_trace_open,
 	.pipe_open	= graph_trace_open,
 	.close		= graph_trace_close,
-- 
1.8.3.2