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Subject[PATCH 10/10] ftrace/x86: Add a counter to test function_graph with direct
From: "Steven Rostedt (VMware)" <rostedt@goodmis.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@goodmis.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

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