test.rb

class Tester
  def foo
    bar
  end

  def bar
    baz
  end

  def baz
    sleep 1
  end
end

t1 = Thread.new do
  Tester.new.foo
end
puts "t1 started"
puts "t1 backtrace: #{t1.backtrace}"
t1.join
puts "t1 joined"

---

t1 started
t1 backtrace: ["test.rb:11:in `sleep'", "test.rb:11:in `baz'", "test.rb:7:in `bar'", "test.rb:3:in `foo'", "test.rb:17:in `block in <main>'"]
t1 joined