Implement preemption of kernel threads natively in the scheduler rather

than as one-off hacks in various other parts of the kernel:
- Add a function maybe_preempt() that is called from sched_add() to
  determine if a thread about to be added to a run queue should be
  preempted to directly.  If it is not safe to preempt or if the new
  thread does not have a high enough priority, then the function returns
  false and sched_add() adds the thread to the run queue.  If the thread
  should be preempted to but the current thread is in a nested critical
  section, then the flag TDF_OWEPREEMPT is set and the thread is added
  to the run queue.  Otherwise, mi_switch() is called immediately and the
  thread is never added to the run queue since it is switch to directly.
  When exiting an outermost critical section, if TDF_OWEPREEMPT is set,
  then clear it and call mi_switch() to perform the deferred preemption.
- Remove explicit preemption from ithread_schedule() as calling
  setrunqueue() now does all the correct work.  This also removes the
  do_switch argument from ithread_schedule().
- Do not use the manual preemption code in mtx_unlock if the architecture
  supports native preemption.
- Don't call mi_switch() in a loop during shutdown to give ithreads a
  chance to run if the architecture supports native preemption since
  the ithreads will just preempt DELAY().
- Don't call mi_switch() from the page zeroing idle thread for
  architectures that support native preemption as it is unnecessary.
- Native preemption is enabled on the same archs that supported ithread
  preemption, namely alpha, i386, and amd64.

This change should largely be a NOP for the default case as committed
except that we will do fewer context switches in a few cases and will
avoid the run queues completely when preempting.

Approved by:	scottl (with his re@ hat)

2 files changed, 3 insertions, 1 deletions

diff --git a/sys/alpha/alpha/interrupt.c b/sys/alpha/alpha/interrupt.c
index 72b833e..d32ce1e 100644
--- a/sys/alpha/alpha/interrupt.c
+++ b/sys/alpha/alpha/interrupt.c
@@ -455,7 +455,7 @@ alpha_dispatch_intr(void *frame, unsigned long vector)
 	 * thread to the current CPU until we return from the interrupt.
 	 */
 	sched_pin();
-	error = ithread_schedule(ithd, !cold);
+	error = ithread_schedule(ithd);
 	KASSERT(error == 0, ("got an impossible stray interrupt"));
 	sched_unpin();
 }
diff --git a/sys/alpha/include/param.h b/sys/alpha/include/param.h
index 1a9c9af..79efec6 100644
--- a/sys/alpha/include/param.h
+++ b/sys/alpha/include/param.h
@@ -113,6 +113,8 @@
 #define	SSIZE		1		/* initial stack size/NBPG */
 #define	SINCR		1		/* increment of stack/NBPG */
 
+#define	PREEMPTION
+
 #ifndef	KSTACK_PAGES
 #define	KSTACK_PAGES	2		/* pages of kstack (with pcb) */
 #endif