sched: spit sched_ap_entry() out of sched_throw()

[kevans]

Summary

sched_throw() can no longer take a NULL thread, APs enter through
sched_ap_entry() instead. This completely removes branching in the
common case. No functional change intended.

[kib]

sys/kern/sched_4bsd.c

1708

I would group this MPASS together with td != NULL

[markj]

sys/kern/sched_ule.c

2987

Maybe explain why: it's important to not enable interrupts until the CPU is running in a threaded context.

sys/sys/sched.h

104

Keep them sorted?

[kevans]

sys/kern/sched_ule.c

2987

Sure, how about (with wrapping fixed):

/*
 * sched_throw_grab() chooses a thread from the queue to switch to
 * next.  It returns with the tdq lock dropped in a spinlock section to
 * keep interrupts disabled until the CPU is running in a proper threaded
 * context.
 */

[markj]

sys/kern/sched_ule.c

2987

Seems ok to me.