Values of WCHAN in ps

Question

What does the WCHAN (waiting channel) mean exactly? what can be its value and what those values represents? I tried to find them but got nothing.

Answer 1

From man ps, section Standard Format Specifiers:

nwchan     WCHAN     address of the kernel function where the process
                     is sleeping (use wchan if you want the kernel
                     function name).  Running tasks will display a
                     dash ('-') in this column.
wchan      WCHAN     name of the kernel function in which the process
                     is sleeping, a "-" if the process is running, or
                     a "*" if the process is multi-threaded and ps is
                     not displaying threads.

As can be seen, they are the kernel functions currently being used by the process. Further:

-n namelist
      Set namelist file.  Identical to N.  The namelist file is needed
      for a proper WCHAN display, and must match the current Linux
      kernel exactly for correct output.  Without this option, the
      default search path for the namelist is:

              $PS_SYSMAP
              $PS_SYSTEM_MAP
              /proc/*/wchan
              /boot/System.map-$(uname -r)
              /boot/System.map
              /lib/modules/$(uname -r)/System.map
              /usr/src/linux/System.map
              /System.map

You can inspect /boot/System.map-$(uname -r) on Ubuntu for a list of functions:

$ sudo head /boot/System.map-$(uname -r)
0000000000000000 D __per_cpu_start
0000000000000000 D irq_stack_union
0000000000000000 A xen_irq_disable_direct_reloc
0000000000000000 A xen_save_fl_direct_reloc
00000000000001e0 A kexec_control_code_size
0000000000004000 d exception_stacks
0000000000009000 D gdt_page
000000000000a000 D espfix_waddr
000000000000a008 D espfix_stack
000000000000a020 D cpu_info

Answer 2

Note that this answer is mostly obsolete: you should never need a namelist or System.map anymore. ps in procps-ng reads /proc/${pid}/wchan directly, instead of reading the 30th field (wchan) out of /proc/${pid}/stat and decoding it against a symbol map file.

In fact, some kernels may set the wchan field of /proc/${pid}/stat to 1 instead of a real value in order to hide the details of kernel address space layout randomization.

(I'm hitting some odd issues with wchan on Fedora 32 though, see fedora bug 1879450).