diff --git a/lib/fluent/root_agent.rb b/lib/fluent/root_agent.rb
index e98ccadd3a..c83f1df976 100644
--- a/lib/fluent/root_agent.rb
+++ b/lib/fluent/root_agent.rb
@@ -136,8 +136,12 @@ def lifecycle(desc: false, kind_callback: nil)
     def start
       lifecycle(desc: true) do |i| # instance
         i.start unless i.started?
-      end
-      lifecycle(desc: true) do |i|
+        # Input#start sometimes emits lots of evetns with in_tail/`read_from_head true` case
+        # and it causes deadlock for small buffer/queue output. To avoid such problem,
+        # buffer related output threads should be run before `Input#start`.
+        # This is why after_start should be called immediately after start call.
+        # This depends on `desc: true` because calling plugin order of `desc: true` is
+        # Output, Filter, Label, Output with Router, then Input.
         i.after_start unless i.after_started?
       end
     end
diff --git a/test/test_plugin_classes.rb b/test/test_plugin_classes.rb
index 09ccf776a1..b2e484c306 100644
--- a/test/test_plugin_classes.rb
+++ b/test/test_plugin_classes.rb
@@ -21,6 +21,28 @@ def shutdown
     end
   end
 
+  class FluentTestGenInput < ::Fluent::Plugin::Input
+    ::Fluent::Plugin.register_input('test_in_gen', self)
+
+    attr_reader :started
+
+    config_param :num, :integer, default: 10000
+
+    def start
+      super
+      @started = true
+
+      @num.times { |i|
+        router.emit("test.evet", Fluent::EventTime.now, {'message' => 'Hello!', 'key' => "value#{i}", 'num' => i})
+      }
+    end
+
+    def shutdown
+      @started = false
+      super
+    end
+  end
+
   class FluentTestOutput < ::Fluent::Plugin::Output
     ::Fluent::Plugin.register_output('test_out', self)
 
@@ -112,6 +134,19 @@ def process(tag, es)
 
   class FluentTestBufferedOutput < ::Fluent::Plugin::Output
     ::Fluent::Plugin.register_output('test_out_buffered', self)
+
+    attr_reader :started
+
+    def start
+      super
+      @started = true
+    end
+
+    def shutdown
+      @started = false
+      super
+    end
+
     def write(chunk)
       # drop everything
     end
diff --git a/test/test_root_agent.rb b/test/test_root_agent.rb
index e979fc0bfc..e467bd81b4 100644
--- a/test/test_root_agent.rb
+++ b/test/test_root_agent.rb
@@ -174,10 +174,15 @@ def configure_ra(conf_str)
   end
 
   sub_test_case 'start/shutdown' do
-    setup do
-      @ra = RootAgent.new(log: $log)
-      stub(Engine).root_agent { @ra }
-      @ra.configure(Config.parse(<<-EOC, "(test)", "(test_dir)", true))
+    def setup_root_agent(conf)
+      ra = RootAgent.new(log: $log)
+      stub(Engine).root_agent { ra }
+      ra.configure(Config.parse(conf, "(test)", "(test_dir)", true))
+      ra
+    end
+
+    test 'plugin status' do
+      ra = setup_root_agent(<<-EOC)
 <source>
   @type test_in
   @id test_in
@@ -191,19 +196,41 @@ def configure_ra(conf_str)
   @id test_out
 </match>
 EOC
-      @ra
+      ra.start
+      assert_true ra.inputs.first.started
+      assert_true ra.filters.first.started
+      assert_true ra.outputs.first.started
+
+      ra.shutdown
+      assert_false ra.inputs.first.started
+      assert_false ra.filters.first.started
+      assert_false ra.outputs.first.started
     end
 
-    test 'plugin status' do
-      @ra.start
-      assert_true @ra.inputs.first.started
-      assert_true @ra.filters.first.started
-      assert_true @ra.outputs.first.started
+    test 'output plugin threads should run before input plugin is blocked with buffer full' do
+      ra = setup_root_agent(<<-EOC)
+<source>
+  @type test_in_gen
+  @id test_in_gen
+</source>
+<match **>
+  @type test_out_buffered
+  @id test_out_buffered
+  <buffer>
+    chunk_limit_size 1k
+    queue_limit_length 2
+    flush_thread_count 2
+    overflow_action block
+  </buffer>
+</match>
+EOC
+      waiting(5) { ra.start }
+      assert_true ra.inputs.first.started
+      assert_true ra.outputs.first.started
 
-      @ra.shutdown
-      assert_false @ra.inputs.first.started
-      assert_false @ra.filters.first.started
-      assert_false @ra.outputs.first.started
+      ra.shutdown
+      assert_false ra.inputs.first.started
+      assert_false ra.outputs.first.started
     end
   end