Enable interrupt preemption on Alpha. Previously, flipping this switch on

caused hangs on SMP systems under load.  My theory was that an interrupted
thread was migrating and returning to PAL on a different CPU and that that
caused the hangs.  To prevent this, I used the recently added sched_pin()
API to pin the interrupted thread to the CPU that received the interrupt
across ithread_schedule() to prevent migration.  This seems to have fixed
the hangs based on tests by several folks on the alpha@ list.

Tested by:	wilko, tisco, several others on alpha@

1 files changed, 10 insertions, 1 deletions

diff --git a/sys/alpha/alpha/interrupt.c b/sys/alpha/alpha/interrupt.c
index 4bce4cd..cccf94a 100644
--- a/sys/alpha/alpha/interrupt.c
+++ b/sys/alpha/alpha/interrupt.c
@@ -47,6 +47,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/malloc.h>
 #include <sys/mutex.h>
 #include <sys/proc.h>
+#include <sys/sched.h>
 #include <sys/smp.h>
 #include <sys/unistd.h>
 #include <sys/vmmeter.h>
@@ -447,8 +448,16 @@ alpha_dispatch_intr(void *frame, unsigned long vector)
 		    "alpha_dispatch_intr: disabling vector 0x%x", i->vector);
 		ithd->it_disable(ithd->it_vector);
 	}
-	error = ithread_schedule(ithd, 0 /* !cold */);
+
+	/*
+	 * It seems that we need to return from an interrupt back to PAL
+	 * on the same CPU that received the interrupt, so pin the interrupted
+	 * thread to the current CPU until we return from the interrupt.
+	 */
+	sched_pin();
+	error = ithread_schedule(ithd, !cold);
 	KASSERT(error == 0, ("got an impossible stray interrupt"));
+	sched_unpin();
 }
 
 static void