build workers are very frequently stuck in this:

[   52s] # expected ''
[   52s] t/IO.t .............
[   52s] Failed 1/18 subtests
[   52s] t/io_const.t ....... ok
[   52s] t/io_dir.t ......... ok
[   52s] t/io_dup.t ......... ok
[   52s] t/io_file.t ........ ok
[   52s] t/io_linenum.t ..... ok
[   53s] t/io_multihomed.t .. ok
[   54s] t/io_pipe.t ........ ok
[   55s] t/io_poll.t ........ ok
[   55s] t/io_sel.t ......... ok
[25790s] [ 8189.601169] BUG: workqueue lockup - pool cpus=0-2 flags=0x4 nice=0
stuck for 25288s!


the BUG is being printed after injecting a keypress, before it is just stuck.
it looks like the guest is not being woken up properly. 

reproduced on a varietey of host kernels including 4.7.0 as a host kernel.