diff --git a/tool/lib/test/unit.rb b/tool/lib/test/unit.rb
index 18b894a893..0eb8ab4c6d 100644
--- a/tool/lib/test/unit.rb
+++ b/tool/lib/test/unit.rb
@@ -406,10 +406,10 @@ module Test
         rescue IOError
         end
 
-        def quit
+        def quit(reason = :normal)
           return if @io.closed?
           @quit_called = true
-          @io.puts "quit"
+          @io.puts "quit #{reason}"
         rescue Errno::EPIPE => e
           warn "#{@pid}:#{@status.to_s.ljust(7)}:#{@file}: #{e.message}"
         end
@@ -534,7 +534,7 @@ module Test
           next unless cond&.call(worker)
           begin
             Timeout.timeout(1) do
-              worker.quit
+              worker.quit(cond ? :timeout : :normal)
             end
           rescue Errno::EPIPE
           rescue Timeout::Error
diff --git a/tool/lib/test/unit/parallel.rb b/tool/lib/test/unit/parallel.rb
index ac297d4a0e..db14ca9d13 100644
--- a/tool/lib/test/unit/parallel.rb
+++ b/tool/lib/test/unit/parallel.rb
@@ -127,7 +127,18 @@ module Test
               else
                 _report "ready"
               end
-            when /^quit$/
+            when /^quit (.+?)$/, "quit"
+              if $1 == "timeout"
+                err = ["", "!!! worker #{$$} killed due to timeout:"]
+                Thread.list.each do |th|
+                  err << "#{ th.inspect }:"
+                  th.backtrace.each do |s|
+                    err << "  #{ s }"
+                  end
+                end
+                err << ""
+                $STDERR.puts err.join("\n")
+              end
               _report "bye"
               exit
             end
diff --git a/tool/test/testunit/test_parallel.rb b/tool/test/testunit/test_parallel.rb
index 6882fd6c5f..371e8524c1 100644
--- a/tool/test/testunit/test_parallel.rb
+++ b/tool/test/testunit/test_parallel.rb
@@ -22,7 +22,7 @@ module TestParallel
       if @worker_pid && @worker_in
         begin
           begin
-            @worker_in.puts "quit"
+            @worker_in.puts "quit normal"
           rescue IOError, Errno::EPIPE
           end
           Timeout.timeout(2) do
@@ -136,7 +136,7 @@ module TestParallel
 
     def test_quit
       Timeout.timeout(TIMEOUT) do
-        @worker_in.puts "quit"
+        @worker_in.puts "quit normal"
         assert_match(/^bye$/m,@worker_out.read)
       end
     end