Basically, I mean the following patch, although I haven't tested it
yet:

Index: locore.s
===================================================================
RCS file: /cvsroot/syssrc/sys/arch/i386/i386/locore.s,v
retrieving revision 1.258
diff -u -r1.258 locore.s
--- locore.s	2002/08/28 09:47:16	1.258
+++ locore.s	2002/10/23 16:28:43
@@ -1667,6 +1667,40 @@
 	.globl	_C_LABEL(sched_whichqs),_C_LABEL(sched_qs)
 	.globl	_C_LABEL(uvmexp),_C_LABEL(panic)
 
+powerof2:
+	.long	0x00000001
+	.long	0x00000002
+	.long	0x00000004
+	.long	0x00000008
+	.long	0x00000010
+	.long	0x00000020
+	.long	0x00000040
+	.long	0x00000080
+	.long	0x00000100
+	.long	0x00000200
+	.long	0x00000400
+	.long	0x00000800
+	.long	0x00001000
+	.long	0x00002000
+	.long	0x00004000
+	.long	0x00008000
+	.long	0x00010000
+	.long	0x00020000
+	.long	0x00040000
+	.long	0x00080000
+	.long	0x00100000
+	.long	0x00200000
+	.long	0x00400000
+	.long	0x00800000
+	.long	0x01000000
+	.long	0x02000000
+	.long	0x04000000
+	.long	0x08000000
+	.long	0x10000000
+	.long	0x20000000
+	.long	0x40000000
+	.long	0x80000000
+
 /*
  * void setrunqueue(struct proc *p);
  * Insert a process on the appropriate queue.  Should be called at splclock().
@@ -1684,9 +1718,10 @@
 	jne	1f
 #endif /* DIAGNOSTIC */
 	movzbl	P_PRIORITY(%eax),%edx
-	shrl	$2,%edx
-	btsl	%edx,_C_LABEL(sched_whichqs)	# set q full bit
-	leal	_C_LABEL(sched_qs)(,%edx,8),%edx # locate q hdr
+	andl	$-4,%edx
+	movl	powerof2(%edx),%ecx
+	orl	%ecx,_C_LABEL(sched_whichqs)	# set q full bit
+	leal	_C_LABEL(sched_qs)(%edx,%edx),%edx # locate q hdr
 	movl	P_BACK(%edx),%ecx
 	movl	%edx,P_FORW(%eax)	# link process on tail of q
 	movl	%eax,P_BACK(%edx)
@@ -1710,9 +1745,10 @@
 	movl	4(%esp),%ecx
 	movzbl	P_PRIORITY(%ecx),%eax
 #ifdef DIAGNOSTIC
-	shrl	$2,%eax
-	btl	%eax,_C_LABEL(sched_whichqs)
-	jnc	1f
+	andl	$-4,%eax
+	movl	powerof2(%eax),%eax
+	andl	_C_LABEL(sched_whichqs),%eax
+	jz	1f
 #endif /* DIAGNOSTIC */
 	movl	P_BACK(%ecx),%edx	# unlink process
 	movl	$0,P_BACK(%ecx)		# zap reverse link to indicate off list
@@ -1722,9 +1758,10 @@
 	cmpl	%ecx,%edx		# q still has something?
 	jne	2f
 #ifndef DIAGNOSTIC
-	shrl	$2,%eax
+	andl	$-4,%eax
+	movl	powerof2(%eax),%eax
 #endif
-	btrl	%eax,_C_LABEL(sched_whichqs)	# no; clear bit
+	subl	%eax,_C_LABEL(sched_whichqs)	# no; clear bit
 2:	ret
 #ifdef DIAGNOSTIC
 1:	pushl	$3f