Provided by: systemtap-doc_2.3-1ubuntu1.4_all 

NAME
function::stack_unused - Returns the amount of kernel stack currently available
SYNOPSIS
stack_unused:long()
ARGUMENTS
None
DESCRIPTION
This function determines how many bytes are currently available in the kernel stack.
SystemTap Tapset Reference March 2016 FUNCTION::STACK_UNUS(3stap)