diff --git a/mercurial/setdiscovery.py b/mercurial/setdiscovery.py
--- a/mercurial/setdiscovery.py
+++ b/mercurial/setdiscovery.py
@@ -241,6 +241,9 @@ def findcommonheads(ui, local, remote,
     elapsed = util.timer() - start
     ui.progress(_('searching'), None)
     ui.debug("%d total queries in %.4fs\n" % (roundtrips, elapsed))
+    msg = 'found %d common and %d missing heads, %d roundtrips in %.4fs\n'
+    ui.log('discovery', msg, len(result), len(srvheadhashes), roundtrips,
+           elapsed)
 
     if not result and srvheadhashes != [nullid]:
         if abortwhenunrelated:
diff --git a/tests/test-setdiscovery.t b/tests/test-setdiscovery.t
--- a/tests/test-setdiscovery.t
+++ b/tests/test-setdiscovery.t
@@ -398,8 +398,13 @@ fixed in 86c35b7ae300:
   > unrandomsample = $TESTTMP/unrandomsample.py
   > EOF
 
-  $ hg -R r1 outgoing r2 -T'{rev} '
+  $ hg -R r1 outgoing r2 -T'{rev} ' --config extensions.blackbox=
   comparing with r2
   searching for changes
   101 102 103 104 105 106 107 108 109 110  (no-eol)
+  $ hg -R r1 --config extensions.blackbox= blackbox
+  * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> outgoing r2 '-T{rev} ' (glob)
+  * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> found 101 common and 101 missing heads, 2 roundtrips in *.????s (glob)
+  * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> -R r1 outgoing r2 '-T{rev} ' --config 'extensions.blackbox=' exited 0 after *.?? seconds (glob)
+  * @5d0b986a083e0d91f116de4691e2aaa54d5bbec0 (*)> blackbox (glob)
   $ cd ..