Red Hat

Here are the different process state codes and description:-

Code:

D    Uninterruptible sleep (usually IO)
R    Running or runnable (on run queue)
S    Interruptible sleep (waiting for an event to complete)
T    Stopped, either by a job control signal or because it is being traced.
W    paging (not valid since the 2.6.xx kernel)
X    dead (should never be seen)
Z    Defunct ("zombie") process, terminated but not reaped by its parent.

As you can see, D means uninterruptible sleep usually due to an IO.

You can check the wchan - name of the kernel function in which the process is sleeping to understand what exactly is going on:-

Code:

ps -eo pid,ppid,state,wchan=WIDE-WCHAN-COLUMN,comm,args | ( read -r; printf "  %s\n" "$REPLY"; grep <your process name/pid> )

Usually it will be a exit_mm() function to release all memory descriptors and related data structures.

As per linux kernel documentation, it first of all checks mm->core_waiters flag is set. If it does, then the process is dumping the contents of memory to a core file (IO). If that is the case, I believe to avoid corruption, it will not respond to a KILL signal until the core file dumping is completed.