ftrace: disable trace events not available in the target kernel

In general we could be interested to define a common configuration to use
across different kernels. Thus, for a given set of experiments, some
trace events can be present only on some kernels and not others.

This patch introduces a check for the availability of trace events by
ensuring that we consider only those available in the kernel in use
in the target. In case of a trace event is not supported in the target
kernel we still log a warning.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>

devlib/trace/ftrace.py

@@ -75,11 +75,12 @@ class FtraceCollector(TraceCollector):
         self.host_binary = None
         self.start_time = None
         self.stop_time = None
-        self.event_string = _build_trace_events(self.events)
+        self.event_string = None
         self.function_string = None
         self._reset_needed = True
 
         # Setup tracing paths
+        self.available_events_file    = self.target.path.join(self.tracing_path, 'available_events')
         self.available_functions_file = self.target.path.join(self.tracing_path, 'available_filter_functions')
         self.buffer_size_file         = self.target.path.join(self.tracing_path, 'buffer_size_kb')
         self.current_tracer_file      = self.target.path.join(self.tracing_path, 'current_tracer')
@@ -104,6 +105,32 @@ class FtraceCollector(TraceCollector):
                 raise TargetError('No trace-cmd found on device and no_install=True is specified.')
             self.target_binary = 'trace-cmd'
 
+        # Validate required events to be traced
+        available_events = self.target.execute(
+                'cat {}'.format(self.available_events_file)).splitlines()
+        selected_events = []
+        for event in self.events:
+            # Convert globs supported by FTrace into valid regexp globs
+            _event = event
+            if event[0] != '*':
+                _event = '*' + event
+            event_re = re.compile(_event.replace('*', '.*'))
+            # Select events matching the required ones
+            if len(filter(event_re.match, available_events)) == 0:
+                message = 'Event [{}] not available for tracing'.format(event)
+                if strict:
+                    raise TargetError(message)
+                self.target.logger.warning(message)
+            else:
+                selected_events.append(event)
+        # If function profiling is enabled we always need at least one event.
+        # Thus, if not other events have been specified, try to add at least
+        # a tracepoint which is always available and possibly triggered few
+        # times.
+        if self.functions and len(selected_events) == 0:
+            selected_events = ['sched_wakeup_new']
+        self.event_string = _build_trace_events(selected_events)
+
         # Check for function tracing support
         if self.functions:
             if not self.target.file_exists(self.function_profile_file):