ftrace: dynamic enabling/disabling of function calls

This patch adds a feature to dynamically replace the ftrace code
with the jmps to allow a kernel with ftrace configured to run
as fast as it can without it configured.

The way this works, is on bootup (if ftrace is enabled), a ftrace
function is registered to record the instruction pointer of all
places that call the function.

Later, if there's still any code to patch, a kthread is awoken
(rate limited to at most once a second) that performs a stop_machine,
and replaces all the code that was called with a jmp over the call
to ftrace. It only replaces what was found the previous time. Typically
the system reaches equilibrium quickly after bootup and there's no code
patching needed at all.

e.g.

  call ftrace  /* 5 bytes */

is replaced with

  jmp 3f  /* jmp is 2 bytes and we jump 3 forward */
3:

When we want to enable ftrace for function tracing, the IP recording
is removed, and stop_machine is called again to replace all the locations
of that were recorded back to the call of ftrace.  When it is disabled,
we replace the code back to the jmp.

Allocation is done by the kthread. If the ftrace recording function is
called, and we don't have any record slots available, then we simply
skip that call. Once a second a new page (if needed) is allocated for
recording new ftrace function calls.  A large batch is allocated at
boot up to get most of the calls there.

Because we do this via stop_machine, we don't have to worry about another
CPU executing a ftrace call as we modify it. But we do need to worry
about NMI's so all functions that might be called via nmi must be
annotated with notrace_nmi. When this code is configured in, the NMI code
will not call notrace.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 18 insertions, 0 deletions

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 740c97dcf9cb..90dbc0ee2046 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -32,6 +32,24 @@ extern void mcount(void);
 # define clear_ftrace_function(ops) do { } while (0)
 #endif /* CONFIG_FTRACE */
 
+#ifdef CONFIG_DYNAMIC_FTRACE
+# define FTRACE_HASHBITS        10
+# define FTRACE_HASHSIZE        (1<<FTRACE_HASHBITS)
+
+struct dyn_ftrace {
+        struct hlist_node node;
+        unsigned long     ip;
+};
+
+/* defined in arch */
+extern struct dyn_ftrace *
+ftrace_alloc_shutdown_node(unsigned long ip);
+extern int ftrace_shutdown_arch_init(void);
+extern void ftrace_code_disable(struct dyn_ftrace *rec);
+extern void ftrace_startup_code(void);
+extern void ftrace_shutdown_code(void);
+extern void ftrace_shutdown_replenish(void);
+#endif
 
 #ifdef CONFIG_FRAME_POINTER
 /* TODO: need to fix this for ARM */