[PATCH] Document backtracer selection options

Signed-off-by: Andi Kleen <ak@suse.de>

Documentation/x86_64/boot-options.txt

@@ -245,6 +245,13 @@ Debugging
 		newfallback: use new unwinder but fall back to old if it gets
 			stuck (default)
 
+  call_trace=[old|both|newfallback|new]
+		old: use old inexact backtracer
+		new: use new exact dwarf2 unwinder
+ 		both: print entries from both
+		newfallback: use new unwinder but fall back to old if it gets
+			stuck (default)
+
 Misc
 
   noreplacement  Don't replace instructions with more appropriate ones