Search notes:
/proc/$pid/wchan
/proc/$pid/wchan contains the symoblic name corresponding to the location in the kernel where the process is sleeping.
wchan stands for wait channel.
The following commands tries to list the possible values in wchan. Note, it was executed on a toy machine and therefore very likely is not complete:
# cd /proc
# paste -d '\n' */wchan | sort | uniq
0
core_sys_select
do_wait
futex_wait_queue_me
hrtimer_nanosleep
pipe_read
put_and_wait_on_page_locked
__x64_sys_poll
__x64_sys_ppoll