ftrace: Set top-level buffer size

trace-cmd start -B devlib -b 42 will set the buffer size for the
"devlib" ftrace instance but will not set the buffer size of the
top-level buffer.

Unfortunately, some events still end up in the top-level buffer
regardless of any configuration such as the "print" event. This can lead
to lost events because the buffer size was too small.

Avoid that by using the buffer size for both top-level and devlib's
instance.

devlib/collector/ftrace.py

@@ -240,6 +240,17 @@ class FtraceCollector(CollectorBase):
     def reset(self):
         self.target.execute('{} reset -B devlib'.format(self.target_binary),
                             as_root=True, timeout=TIMEOUT)
+
+        # trace-cmd start will not set the top-level buffer size if passed -B
+        # parameter, but unfortunately some events still end up there (e.g.
+        # print event). So we still need to set that size, otherwise the buffer
+        # might be too small and some event lost.
+        if self.buffer_size is not None:
+            self.target.write_value(
+                self.target.path.join(self.tracing_path, 'buffer_size_kb'),
+                self.buffer_size
+            )
+
         if self.functions:
             self.target.write_value(self.function_profile_file, 0, verify=False)
         self._reset_needed = False