[for-linus][PATCH 6/7] ftrace: Make ftrace_graph_is_dead() a static branch

Steven Rostedt
2022-04-03T14:32:05Z

From: Christophe Leroy <christophe.leroy@csgroup.eu>

ftrace_graph_is_dead() is used on hot paths, it just reads a variable
in memory and is not worth suffering function call constraints.

For instance, at entry of prepare_ftrace_return(), inlining it avoids
saving prepare_ftrace_return() parameters to stack and restoring them
after calling ftrace_graph_is_dead().

While at it using a static branch is even more performant and is
rather well adapted considering that the returned value will almost
never change.

Inline ftrace_graph_is_dead() and replace 'kill_ftrace_graph' bool
by a static branch.

The performance improvement is noticeable.

Link: https://lkml.kernel.org/r/e0411a6a0ed3eafff0ad2bc9cd4b0e202b4617df.1648623570.git.christophe.leroy@csgroup.eu

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
include/linux/ftrace.h | 16 +++++++++++++++-
kernel/trace/fgraph.c | 17 +++--------------
2 files changed, 18 insertions(+), 15 deletions(-)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 37b619185ec9..f15a4b76cbfc 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -9,6 +9,7 @@

#include <linux/trace_recursion.h>
#include <linux/trace_clock.h>
+#include <linux/jump_label.h>
#include <linux/kallsyms.h>
#include <linux/linkage.h>
#include <linux/bitops.h>
@@ -1015,7 +1016,20 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
extern int register_ftrace_graph(struct fgraph_ops *ops);
extern void unregister_ftrace_graph(struct fgraph_ops *ops);

-extern bool ftrace_graph_is_dead(void);
+/**
+ * ftrace_graph_is_dead - returns true if ftrace_graph_stop() was called
+ *
+ * ftrace_graph_stop() is called when a severe error is detected in
+ * the function graph tracing. This function is called by the critical
+ * paths of function graph to keep those paths from doing any more harm.
+ */
+DECLARE_STATIC_KEY_FALSE(kill_ftrace_graph);
+
+static inline bool ftrace_graph_is_dead(void)
+{
+ return static_branch_unlikely(&kill_ftrace_graph);
+}
+
extern void ftrace_graph_stop(void);

/* The current handlers in use */
diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index 19028e072cdb..8f4fb328133a 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -7,6 +7,7 @@
*
* Highly modified by Steven Rostedt (VMware).
*/
+#include <linux/jump_label.h>
#include <linux/suspend.h>
#include <linux/ftrace.h>
#include <linux/slab.h>
@@ -23,24 +24,12 @@
#define ASSIGN_OPS_HASH(opsname, val)
#endif

-static bool kill_ftrace_graph;
+DEFINE_STATIC_KEY_FALSE(kill_ftrace_graph);
int ftrace_graph_active;

/* Both enabled by default (can be cleared by function_graph tracer flags */
static bool fgraph_sleep_time = true;

-/**
- * ftrace_graph_is_dead - returns true if ftrace_graph_stop() was called
- *
- * ftrace_graph_stop() is called when a severe error is detected in
- * the function graph tracing. This function is called by the critical
- * paths of function graph to keep those paths from doing any more harm.
- */
-bool ftrace_graph_is_dead(void)
-{
- return kill_ftrace_graph;
-}
-
/**
* ftrace_graph_stop - set to permanently disable function graph tracing
*
@@ -51,7 +40,7 @@ bool ftrace_graph_is_dead(void)
*/
void ftrace_graph_stop(void)
{
- kill_ftrace_graph = true;
+ static_branch_enable(&kill_ftrace_graph);
}

/* Add a function return address to the trace stack on thread info.*/
--
2.35.1