tracing: Add snapshot at end of kernel boot up

Add ftrace_boot_snapshot kernel parameter that will take a snapshot at the
end of boot up just before switching over to user space (it happens during
the kernel freeing of init memory).

This is useful when there's interesting data that can be collected from
kernel start up, but gets overridden by user space start up code. With
this option, the ring buffer content from the boot up traces gets saved in
the snapshot at the end of boot up. This trace can be read from:

 /sys/kernel/tracing/snapshot

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 10 insertions, 1 deletions

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 9999e29187de..37b619185ec9 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -30,6 +30,12 @@
 #define ARCH_SUPPORTS_FTRACE_OPS 0
 #endif
 
+#ifdef CONFIG_TRACING
+extern void ftrace_boot_snapshot(void);
+#else
+static inline void ftrace_boot_snapshot(void) { }
+#endif
+
 #ifdef CONFIG_FUNCTION_TRACER
 struct ftrace_ops;
 struct ftrace_regs;
@@ -215,7 +221,10 @@ struct ftrace_ops_hash {
 void ftrace_free_init_mem(void);
 void ftrace_free_mem(struct module *mod, void *start, void *end);
 #else
-static inline void ftrace_free_init_mem(void) { }
+static inline void ftrace_free_init_mem(void)
+{
+	ftrace_boot_snapshot();
+}
 static inline void ftrace_free_mem(struct module *mod, void *start, void *end) { }
 #endif