linux-kernel - [PATCH v1] ftrace: Make ftrace_graph_is

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Date:   Thu, 24 Mar 2022 11:01:45 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v1] ftrace: Make ftrace_graph_is_dead() static inline

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().

The performance improvement is noticeable.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 include/linux/ftrace.h | 15 ++++++++++++++-
 kernel/trace/fgraph.c  | 14 +-------------
 2 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 9999e29187de..cecdda515059 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -1006,7 +1006,20 @@ 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.
+ */
+extern bool kill_ftrace_graph;
+
+static inline bool ftrace_graph_is_dead(void)
+{
+	return 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 22061d38fc00..3961f5884063 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -23,24 +23,12 @@
 #define ASSIGN_OPS_HASH(opsname, val)
 #endif
 
-static bool kill_ftrace_graph;
+bool 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
  *
-- 
2.35.1