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)

1 files changed, 1 insertions, 1 deletions

diff --git a/sys/sys/interrupt.h b/sys/sys/interrupt.h
index 7f7ddef..28d6413 100644
--- a/sys/sys/interrupt.h
+++ b/sys/sys/interrupt.h
@@ -122,7 +122,7 @@ int	ithread_add_handler(struct ithd *ithread, const char *name,
 	    driver_intr_t handler, void *arg, u_char pri, enum intr_type flags,
 	    void **cookiep);
 int	ithread_remove_handler(void *cookie);
-int	ithread_schedule(struct ithd *ithread, int do_switch);
+int	ithread_schedule(struct ithd *ithread);
 int     swi_add(struct ithd **ithdp, const char *name,
 	    driver_intr_t handler, void *arg, int pri, enum intr_type flags,
 	    void **cookiep);