collector/ftrace: Make emitting cpu_frequency_devlib and extra idle events dependent on the configured events

Make cpu_frequency_devlib dependent on whether the "cpu_frequency" event
has been selected rather than dependent on the cpufreq devlib module
being loaded on the target.

The old behavior became particularly problematic with the lazy loading
of modules. However, it was never a reliable way of knowing if the user
was interested in the frequency or not.

Apply a similar mechanism for the extra idle state transitions only done
if "cpu_idle" event is selected.

Author: douglas-raillard-arm

devlib/collector/ftrace.py

@@ -93,7 +93,6 @@ def __init__(self, target,
         self.host_binary = None
         self.start_time = None
         self.stop_time = None
-        self.event_string = None
         self.function_string = None
         self.trace_clock = trace_clock
         self.saved_cmdlines_nr = saved_cmdlines_nr
@@ -195,7 +194,11 @@ def event_is_in_list(event, events):
             elif self.tracer == 'function_graph':
                 self.function_string = _build_graph_functions(selected_functions, trace_children_functions)
 
-        self.event_string = _build_trace_events(selected_events)
+        self._selected_events = selected_events
+
+    @property
+    def event_string(self):
+        return _build_trace_events(self._selected_events)
 
     @classmethod
     def _resolve_tracing_path(cls, target, path):
@@ -270,6 +273,26 @@ def reset(self):
 
         self._reset_needed = False
 
+    def _trace_frequencies(self):
+        if 'cpu_frequency' in self._selected_events:
+            self.logger.debug('Trace CPUFreq frequencies')
+            try:
+                mod = self.target.cpufreq
+            except TargetStableError as e:
+                self.logger.error(f'Could not trace CPUFreq frequencies as the cpufreq module cannot be loaded: {e}')
+            else:
+                mod.trace_frequencies()
+
+    def _trace_idle(self):
+        if 'cpu_idle' in self._selected_events:
+            self.logger.debug('Trace CPUIdle states')
+            try:
+                mod = self.target.cpuidle
+            except TargetStableError as e:
+                self.logger.error(f'Could not trace CPUIdle states as the cpuidle module cannot be loaded: {e}')
+            else:
+                mod.perturb_cpus()
+
     @asyncf
     async def start(self):
         self.start_time = time.time()
@@ -302,12 +325,10 @@ async def start(self):
         )
         if self.automark:
             self.mark_start()
-        if 'cpufreq' in self.target.modules:
-            self.logger.debug('Trace CPUFreq frequencies')
-            self.target.cpufreq.trace_frequencies()
-        if 'cpuidle' in self.target.modules:
-            self.logger.debug('Trace CPUIdle states')
-            self.target.cpuidle.perturb_cpus()
+
+        self._trace_frequencies()
+        self._trace_idle()
+
         # Enable kernel function profiling
         if self.functions and self.tracer is None:
             target = self.target