It's because for a long time, there was only "interruptible" and "uninterruptible", where "interruptible" meant that any signal could interrupt it. Historicall

It's because for a long time, there was only "interruptible" and "uninterruptible", where "interruptible" meant that any signal could interrupt it. Historically, many operations were specified not to be interrupted by signals - reads and writes on disk files (as opposed to "slow devices" like ttys and sockets), mkdir(2), fsync(2), etc. They're not allowed to return EINTR. This meant using an uninterruptible sleep, which has the consequence that SIGKILL can't interrupt it either. That's why the code in top or ps for uninterruptible sleep is 'D' - historically it meant "Disk Wait". TASK_KILLABLE is a more recent invention, which can be interrupted only by fatal signals. The code doing the wait still has to unwind as if it were going to exit from the syscall with an error, but userspace won't see that error because it will die from the signal instead.