ftrace: dump out ftrace buffers to console on panic

At OLS I had a lot of interest to be able to have the ftrace buffers
dumped on panic.  Usually one would expect to uses kexec and examine
the buffers after a new kernel is loaded. But sometimes the resources
do not permit kdump and kexec, so having an option to still see the
sequence of events up to the crash is very advantageous.

This patch adds the option to have the ftrace buffers dumped to the
console in the latency_trace format on a panic. When the option is set,
the default entries per CPU buffer are lowered to 16384, since the writing
to the serial (if that is the console) may take an awful long time
otherwise.

[
 Changes since -v1:
  Got alpine to send correctly (as well as spell check working).
  Removed config option.
  Moved the static variables into ftrace_dump itself.
  Gave printk a log level.
]

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 018af16bce5c..f7fb92045bf0 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -178,6 +178,7 @@ ftrace_special(unsigned long arg1, unsigned long arg2, unsigned long arg3);
 extern int
 __ftrace_printk(unsigned long ip, const char *fmt, ...)
         __attribute__ ((format (printf, 2, 3)));
+extern void ftrace_dump(void);
 #else
 static inline void
 ftrace_special(unsigned long arg1, unsigned long arg2, unsigned long arg3) { }
@@ -186,6 +187,7 @@ ftrace_printk(const char *fmt, ...) __attribute__ ((format (printf, 1, 0)))
 {
         return 0;
 }
+static inline void ftrace_dump(void) { }
 #endif
 
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD