Forget it pls, here I am wrong, sorry for pestering.
But as I observe by cat /proc/sched_debug (2.6.21.1, UP, RHEL4), I found the all waiting fields often are more than zero, or less than zero.
IMHO, the sum of task_struct->wait_runtime just is the denominator of all runnable time in some ways, is it right? if so, increasing the sum of wait_runtime just make scheduling decision more precise. so what's meaning for keeping the wait_runtime is zero-sum?