This is worth a whopping 2% on spwecweb on an 8-way.  Which is faintly
surprising because __wake_up and other wait/wakeup functions are not
apparent in the specweb profiles which I've seen.


The main objective of this is to reduce the CPU cost of the wait/wakeup
operation.  When a task is woken up, its waitqueue is removed from the
waitqueue_head by the waker (ie: immediately), rather than by the woken
process.

This means that a subsequent wakeup does not need to revisit the
just-woken task.  It also means that the just-woken task does not need
to take the waitqueue_head's lock, which may well reside in another
CPU's cache.

I have no decent measurements on the effect of this change - possibly a
20-30% drop in __wake_up cost in Badari's 40-dds-to-40-disks test (it
was the most expensive function), but it's inconclusive.  And no
quantitative testing of which I am aware has been performed by
networking people.

The API is very simple to use (Linus thought it up):

my_func(waitqueue_head_t *wqh)
{
	DEFINE_WAIT(wait);

	prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
	if (!some_test)
		schedule();
	finish_wait(wqh, &wait);
}

or:

	DEFINE_WAIT(wait);

	while (!some_test_1) {
		prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
		if (!some_test_2)
			schedule();
		...
	}
	finish_wait(wqh, &wait);

You need to bear in mind that once prepare_to_wait has been performed,
your task could be removed from the waitqueue_head and placed into
TASK_RUNNING at any time.  You don't know whether or not you're still
on the waitqueue_head.

Running prepare_to_wait() when you're already on the waitqueue_head is
fine - it will do the right thing.

Running finish_wait() when you're actually not on the waitqueue_head is
fine.

Running finish_wait() when you've _never_ been on the waitqueue_head is
fine, as ling as the DEFINE_WAIT() macro was used to initialise the
waitqueue.

You don't need to fiddle with current->state.  prepare_to_wait() and
finish_wait() will do that.  finish_wait() will always return in state
TASK_RUNNING.

There are plenty of usage examples in vm-wakeups.patch and
tcp-wakeups.patch.