linux-kernel - [PATCH 10/10] ftrace/x86: Add a counter to test function

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Date:   Fri, 08 Nov 2019 16:28:44 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org
Cc:     Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        X86 ML <x86@...nel.org>, Nadav Amit <nadav.amit@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Song Liu <songliubraving@...com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: [PATCH 10/10] ftrace/x86: Add a counter to test function_graph with direct

From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>

As testing for direct calls from the function graph tracer adds a little
overhead (which is a lot when tracing every function), add a counter that
can be used to test if function_graph tracer needs to test for a direct
caller or not.

It would have been nicer if we could use a static branch, but the static
branch logic fails when used within the function graph tracer trampoline.

Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 arch/x86/kernel/ftrace.c | 8 +++++---
 include/linux/ftrace.h   | 2 ++
 kernel/trace/ftrace.c    | 4 ++++
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index fef283f6341d..060a361d9d11 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -1049,9 +1049,11 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
 	 * return address is actually off by one word, and we
 	 * need to adjust for that.
 	 */
-	if (ftrace_find_direct_func(self_addr + MCOUNT_INSN_SIZE)) {
-		self_addr = *parent;
-		parent++;
+	if (ftrace_direct_func_count) {
+		if (ftrace_find_direct_func(self_addr + MCOUNT_INSN_SIZE)) {
+			self_addr = *parent;
+			parent++;
+		}
 	}
 
 	/*
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 2bc7bd6b8387..55647e185141 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -247,10 +247,12 @@ static inline void ftrace_free_mem(struct module *mod, void *start, void *end) {
 #endif /* CONFIG_FUNCTION_TRACER */
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+extern int ftrace_direct_func_count;
 int register_ftrace_direct(unsigned long ip, unsigned long addr);
 int unregister_ftrace_direct(unsigned long ip, unsigned long addr);
 struct ftrace_direct_func *ftrace_find_direct_func(unsigned long addr);
 #else
+# define ftrace_direct_func_count 0
 static inline int register_ftrace_direct(unsigned long ip, unsigned long addr)
 {
 	return -ENODEV;
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index f57ab704dc2d..0e18ec707a88 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -2367,6 +2367,7 @@ ftrace_find_tramp_ops_new(struct dyn_ftrace *rec)
 /* Protected by rcu_tasks for reading, and direct_mutex for writing */
 static struct ftrace_hash *direct_functions = EMPTY_HASH;
 static DEFINE_MUTEX(direct_mutex);
+int ftrace_direct_func_count;
 
 /*
  * Search the direct_functions hash to see if the given instruction pointer
@@ -5059,6 +5060,7 @@ int register_ftrace_direct(unsigned long ip, unsigned long addr)
 		direct->addr = addr;
 		direct->count = 0;
 		list_add_rcu(&direct->next, &ftrace_direct_funcs);
+		ftrace_direct_func_count++;
 	}
 
 	entry->ip = ip;
@@ -5084,6 +5086,7 @@ int register_ftrace_direct(unsigned long ip, unsigned long addr)
 			if (free_hash)
 				free_ftrace_hash(free_hash);
 			free_hash = NULL;
+			ftrace_direct_func_count--;
 		}
 	} else {
 		if (!direct->count)
@@ -5144,6 +5147,7 @@ int unregister_ftrace_direct(unsigned long ip, unsigned long addr)
 			list_del_rcu(&direct->next);
 			synchronize_rcu_tasks();
 			kfree(direct);
+			ftrace_direct_func_count--;
 		}
 	}
  out_unlock:
-- 
2.23.0