Index: sys/i386/include/proc.h
===================================================================
--- sys/i386/include/proc.h
+++ sys/i386/include/proc.h
@@ -74,7 +74,7 @@
 /* Get the current kernel thread stack usage. */
 #define GET_STACK_USAGE(total, used) do {				\
 	struct thread	*td = curthread;				\
-	(total) = td->td_kstack_pages * PAGE_SIZE;			\
+	(total) = td->td_kstack_pages * PAGE_SIZE - sizeof(struct pcb);	\
 	(used) = (char *)td->td_kstack +				\
 	    td->td_kstack_pages * PAGE_SIZE -				\
 	    (char *)&td;						\