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, 2 insertions, 0 deletions

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index f9feb197b2da..4e29bd1cf151 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -7096,6 +7096,8 @@ void __init ftrace_free_init_mem(void)
 	void *start = (void *)(&__init_begin);
 	void *end = (void *)(&__init_end);
 
+	ftrace_boot_snapshot();
+
 	ftrace_free_mem(NULL, start, end);
 }