Debug stuck unicorn threads

unicorn_killer.rb

after_fork do |server, worker|
  Signal.trap('USR2') do
    pid = Process.pid 
    puts "[#{pid}] Received USR2 at #{Time.now}. Dumping threads:"
    Thread.list.each do |t|
      trace = t.backtrace.join("\n[#{pid}] ")
      puts "[#{pid}] #{trace}"
      puts "[#{pid}] ---"
    end
    puts "[#{pid}] -------------------"
  end
end

Source: http://varaneckas.com/blog/ruby-tracing-threads-unicorn/