sched_ule vs un-spun-up AP

[Chris Torek]

This patch is possibly a bit of unnecessary fluff, but it fixes a
panic we see with some rather odd code (said odd code needs to be
redone) that tries to bind a thread to a CPU during startup.

On real hardware, the APs are up by this point and have idle
threads.

In a bhyve emulation, however, the APs spin up much later,
relatively speaking, so that when we reach the tdq_notify() code
in sched_ule.c, the target CPU on which the bound thread is
supposed to run is not up yet.  pcpu_find(cpu) works fine but
its pc_curthread is NULL.  The test to see if the new thread
should run instead, causes this:

    Fatal trap 12: page fault while in kernel mode
    cpuid = 0; apic id = 00
    fault virtual address   = 0x356
    fault code              = supervisor read data, page not present
    instruction pointer     = 0x20:0xffffffff805aaaf2
    stack pointer           = 0x28:0xfffff800002a98c0
    frame pointer           = 0x28:0xfffff800002a98f0
    code segment            = base 0x0, limit 0xfffff, type 0x1b
			    = DPL 0, pres 1, long 1, def32 0, gran 1
    processor eflags        = resume, IOPL = 0
    current process         = 3 (evil-bound-thread)
    [ thread pid 3 tid 100022 ]
    Stopped at      tdq_notify+0x32:        movb    0x356(%rax),%cl
    db> 

A one-line patch works around it.  A better fix would probably be
to make sure the APs are up and running earlier, but I stuck the
below in so that our guys could make progress on their code while
we try to fix this other sort-of-evil early bound thread.

Chris

sched_ule: in tdq_notify, handle un-spun-up CPU

If a thread tries to bind to a CPU that is present on an SMP system,
but has yet to spin up, don't try to look at the thread priority
on that CPU, as there is no thread (not even the idle thread)
running.


diff --git a/sys/kern/sched_ule.c b/sys/kern/sched_ule.c
--- a/sys/kern/sched_ule.c
+++ b/sys/kern/sched_ule.c
@@ -1004,7 +1004,12 @@ tdq_notify(struct tdq *tdq, struct threa
 	cpu = td->td_sched->ts_cpu;
 	pri = td->td_priority;
 	ctd = pcpu_find(cpu)->pc_curthread;
-	if (!sched_shouldpreempt(pri, ctd->td_priority, 1))
+	/*
+	 * Note: ctd==NULL occurs only when binding to a cpu
+	 * that has not yet finished coming up (so it has no
+	 * idle thread).
+	 */
+	if (ctd == NULL || !sched_shouldpreempt(pri, ctd->td_priority, 1))
 		return;
 	if (TD_IS_IDLETHREAD(ctd)) {
 		/*