perf top: Add callgraph support

Just like in 'perf report', but live.

Still needs to decay the callchains, but already somewhat useful as-is.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-cj3rmaf5jpsvi3v0tf7t4uvp@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-top.txt

@@ -126,6 +126,21 @@ Default is to monitor all CPUS.
 --symbols::
 	Only consider these symbols.
 
+-G [type,min,order]::
+--call-graph::
+        Display call chains using type, min percent threshold and order.
+	type can be either:
+	- flat: single column, linear exposure of call chains.
+	- graph: use a graph tree, displaying absolute overhead rates.
+	- fractal: like graph, but displays relative rates. Each branch of
+		 the tree is considered as a new profiled object.
+
+	order can be either:
+	- callee: callee based call graph.
+	- caller: inverted caller based call graph.
+
+	Default: fractal,0.5,callee.
+
 INTERACTIVE PROMPTING KEYS
 --------------------------