trace: strip leading unimportant frames from backtrace

First couple of frames always come from trace logging functions. I
think we should not show them because they do not add any information.

There was already a list of function names which should be filtered
out. This is not appropriate in this case because tracing goes through
a function named 'log' which is a pretty generic name and could as
well be an application symbol, so we cannot just filter out all
functions named 'log. This patch adds support for knocking off a
leading sequence of frames form the backtrace which is more accurate
and seems good enough.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

scripts/osv/prof.py

@@ -63,15 +63,17 @@ def format_time(time, format="%.2f %s"):
     return str(time)
 
 unimportant_functions = set([
-    'trace_slow_path',
     'operator()',
     'std::function<void ()>::operator()() const',
-    'tracepoint_base::do_log_backtrace',
-    'tracepoint_base::log_backtrace(trace_record*, unsigned char*&)',
-    'tracepoint_base::do_log_backtrace(trace_record*, unsigned char*&)',
     '_M_invoke',
     ])
 
+unimportant_prefixes = [
+    ('tracepoint_base::log_backtrace(trace_record*, unsigned char*&)',
+     'log',
+     'trace_slow_path'),
+]
+
 bottom_of_stack = set(['thread_main', 'thread_main_c'])
 
 def strip_garbage(backtrace):
@@ -80,6 +82,12 @@ def strip_garbage(backtrace):
             return True
         return not src_addr.name in unimportant_functions
 
+    for chain in unimportant_prefixes:
+        if len(backtrace) >= len(chain) and \
+                tuple(map(attrgetter('name'), backtrace[:len(chain)])) == chain:
+            backtrace = backtrace[len(chain):]
+            break
+
     backtrace = list(filter(is_good, backtrace))
 
     for i, src_addr in enumerate(backtrace):