diff -r e3501546f7e4 -r 88d3c1ab03a7 mercurial/profiling.py
--- a/mercurial/profiling.py	Sun Aug 14 17:51:12 2016 -0700
+++ b/mercurial/profiling.py	Sun Aug 14 18:28:43 2016 -0700
@@ -88,7 +88,9 @@
 
     freq = ui.configint('profiling', 'freq', default=1000)
     if freq > 0:
-        statprof.reset(freq)
+        # Cannot reset when profiler is already active. So silently no-op.
+        if statprof.state.profile_level == 0:
+            statprof.reset(freq)
     else:
         ui.warn(_("invalid sampling frequency '%s' - ignoring\n") % freq)