diff --git a/History.md b/History.md
index 2fab78af9a..667ff59a91 100644
--- a/History.md
+++ b/History.md
@@ -24,6 +24,7 @@
   * Rescue IO::WaitReadable instead of EAGAIN for blocking read (#2121)
   * Ensure `BUNDLE_GEMFILE` is unspecified in workers if unspecified in master when using `prune_bundler` (#2154)
   * Rescue and log exceptions in hooks defined by users (on_worker_boot, after_worker_fork etc) (#1551)
+  * Fix `out_of_band` hook never executed if the number of worker threads is > 1 (#2177)
   
 * Refactor
   * Remove unused loader argument from Plugin initializer (#2095)
diff --git a/lib/puma/server.rb b/lib/puma/server.rb
index fc5e24fd55..8ca3fabb87 100644
--- a/lib/puma/server.rb
+++ b/lib/puma/server.rb
@@ -63,6 +63,9 @@ def initialize(app, events=Events.stdio, options={})
 
       @status = :stop
 
+      @all_worker_threads_free = false
+      @all_worker_threads_free_mutex = Mutex.new
+
       @min_threads = 0
       @max_threads = 16
       @auto_trim_time = 30
@@ -189,6 +192,7 @@ def run(background=true)
 
       @thread_pool = ThreadPool.new(@min_threads,
                                     @max_threads,
+                                    Proc.new { signal_all_worker_threads_free },
                                     ::Puma::IOBuffer) do |client, buffer|
 
         # Advertise this server into the thread
@@ -290,10 +294,6 @@ def handle_servers
                     end
 
                     pool << client
-                    busy_threads = pool.wait_until_not_full
-                    if busy_threads == 0
-                      @options[:out_of_band].each(&:call) if @options[:out_of_band]
-                    end
                   end
                 rescue SystemCallError
                   # nothing
@@ -310,6 +310,19 @@ def handle_servers
           rescue Object => e
             @events.unknown_error self, e, "Listen loop"
           end
+
+          if @all_worker_threads_free_mutex.try_lock # Do not block the critical path if mutex isn't available
+            if @all_worker_threads_free
+              begin
+                @options[:out_of_band].each(&:call) if @options[:out_of_band]
+              rescue Exception => e
+                STDERR.puts "Exception handling OOB callbacks: #{e.message} (#{e.class})"
+                STDERR.puts e.backtrace
+              end
+              @all_worker_threads_free = false
+            end
+            @all_worker_threads_free_mutex.unlock
+          end
         end
 
         @events.fire :state, @status
@@ -344,6 +357,8 @@ def handle_check
       when RESTART_COMMAND
         @status = :restart
         return true
+      when NOTIFY_THREADS_FREE_COMMAND
+        @all_worker_threads_free = true
       end
 
       return false
@@ -945,5 +960,12 @@ def possible_header_injection?(header_value)
       HTTP_INJECTION_REGEX =~ header_value.to_s
     end
     private :possible_header_injection?
+
+    def signal_all_worker_threads_free
+      @all_worker_threads_free_mutex.synchronize do
+        @all_worker_threads_free = true
+      end
+    end
+    private :signal_all_worker_threads_free
   end
 end
diff --git a/lib/puma/thread_pool.rb b/lib/puma/thread_pool.rb
index 85ba68c96e..d90cf42189 100644
--- a/lib/puma/thread_pool.rb
+++ b/lib/puma/thread_pool.rb
@@ -29,7 +29,7 @@ class ForceShutdown < RuntimeError
     # The block passed is the work that will be performed in each
     # thread.
     #
-    def initialize(min, max, *extra, &block)
+    def initialize(min, max, all_threads_free_cb, *extra, &block)
       @not_empty = ConditionVariable.new
       @not_full = ConditionVariable.new
       @mutex = Mutex.new
@@ -41,6 +41,7 @@ def initialize(min, max, *extra, &block)
 
       @min = Integer(min)
       @max = Integer(max)
+      @all_threads_free_cb = all_threads_free_cb
       @block = block
       @extra = extra
 
@@ -116,6 +117,7 @@ def spawn_thread
               end
 
               @waiting += 1
+              signal_threadpool_empty
               not_full.signal
               not_empty.wait mutex
               @waiting -= 1
@@ -213,6 +215,15 @@ def wait_until_not_full
       end
     end
 
+    # Call the callback +@all_threads_free_cb+ if all worker threads are free.
+    #
+    def signal_threadpool_empty
+      busy_threads = @spawned - @waiting
+      if busy_threads == 0
+        @all_threads_free_cb.call
+      end
+    end
+
     # If too many threads are in the pool, tell one to finish go ahead
     # and exit. If +force+ is true, then a trim request is requested
     # even if all threads are being utilized.
diff --git a/test/test_thread_pool.rb b/test/test_thread_pool.rb
index 18901d6a45..4815aca61a 100644
--- a/test/test_thread_pool.rb
+++ b/test/test_thread_pool.rb
@@ -10,9 +10,10 @@ def teardown
 
   def new_pool(min, max, &block)
     block = proc { } unless block
+    all_worker_threads_free_cb = proc { }
     @work_mutex = Mutex.new
     @work_done = ConditionVariable.new
-    @pool = Puma::ThreadPool.new(min, max, &block)
+    @pool = Puma::ThreadPool.new(min, max, all_worker_threads_free_cb, &block)
   end
 
   def pause