Getting a Ruby thread dump

C stack trace:

gdb -p <RUBY_PID>
(gdb) thread apply all backtrace
(gdb) quit

Please note that while gdb is attached the Ruby process is blocked.

Ruby stack trace:

gdb -p <RUBY_PID>
(gdb) call (void) close(1)
(gdb) call (void) close(2)
(gdb) shell tty
/dev/pts/0
(gdb) call (int) open("/dev/pts/0", 2, 0)
$1 = 1
(gdb) call (int) open("/dev/pts/0", 2, 0)
$2 = 2
(gdb) call (void)rb_backtrace()
(gdb) quit

It is possible to script gdb by putting commands in a file passed with the -x option. Note that this file should contain set pagination off as the first instruction.

Full details about the technique: https://blog.newrelic.com/engineering/debugging-stuck-ruby-processes-what-to-do-before-you-kill-9/