[PATCH] 2.5: preemptive kernel on UP

[Robert Love]

During 2.5.5-pre, an optimization was made that removed schedule_tail
from UP kernels.  This causes the initial preempt_count of a new task,
which starts at 1, to never decrement to zero and thus never become
preemptible.

Thanks to everyone who pointed out the lousy performance - it took
awhile to find but it should be fixed now.  Please comment if not.

Patch is against 2.5.6-pre2 and is critical for all UP+preempt users.

	Robert Love

diff -urN linux-2.5.6-pre2/arch/alpha/kernel/entry.S linux/arch/alpha/kernel/entry.S
--- linux-2.5.6-pre2/arch/alpha/kernel/entry.S	Fri Mar  1 17:21:14 2002
+++ linux/arch/alpha/kernel/entry.S	Mon Mar  4 17:49:27 2002
@@ -495,7 +495,7 @@
 	ret	$31,($26),1
 .end alpha_switch_to
 
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
 .globl  ret_from_fork
 .align 3
 .ent ret_from_fork
diff -urN linux-2.5.6-pre2/arch/i386/kernel/entry.S linux/arch/i386/kernel/entry.S
--- linux-2.5.6-pre2/arch/i386/kernel/entry.S	Tue Feb 19 21:10:58 2002
+++ linux/arch/i386/kernel/entry.S	Mon Mar  4 17:48:32 2002
@@ -195,7 +195,7 @@
 

 ENTRY(ret_from_fork)
-#if CONFIG_SMP
+#if CONFIG_SMP || CONFIG_PREEMPT
 	call SYMBOL_NAME(schedule_tail)
 #endif
 	GET_THREAD_INFO(%ebx)
diff -urN linux-2.5.6-pre2/arch/ppc/kernel/entry.S linux/arch/ppc/kernel/entry.S
--- linux-2.5.6-pre2/arch/ppc/kernel/entry.S	Tue Feb 19 21:10:59 2002
+++ linux/arch/ppc/kernel/entry.S	Mon Mar  4 17:48:41 2002
@@ -343,7 +343,7 @@
 
 	.globl	ret_from_fork
 ret_from_fork:
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
 	bl	schedule_tail
 #endif
 	rlwinm	r3,r1,0,0,18
diff -urN linux-2.5.6-pre2/arch/ppc64/kernel/entry.S linux/arch/ppc64/kernel/entry.S
--- linux-2.5.6-pre2/arch/ppc64/kernel/entry.S	Tue Feb 19 21:10:53 2002
+++ linux/arch/ppc64/kernel/entry.S	Mon Mar  4 17:52:16 2002
@@ -311,7 +311,7 @@
 	blr
 
 _GLOBAL(ret_from_fork)
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
 	bl	.schedule_tail
 #endif
 	clrrdi	r4,r1,THREAD_SHIFT
diff -urN linux-2.5.6-pre2/arch/s390/kernel/entry.S linux/arch/s390/kernel/entry.S
--- linux-2.5.6-pre2/arch/s390/kernel/entry.S	Tue Feb 19 21:10:57 2002
+++ linux/arch/s390/kernel/entry.S	Mon Mar  4 17:48:12 2002
@@ -295,7 +295,7 @@
         stosm   24(%r15),0x03     # reenable interrupts
         sr      %r0,%r0           # child returns 0
         st      %r0,SP_R2(%r15)   # store return value (change R2 on stack)
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
         l       %r1,BASED(.Lschedtail)
 	la      %r14,BASED(sysc_return)
         br      %r1               # call schedule_tail, return to sysc_return
@@ -896,7 +896,7 @@
 #error .Ltrace:       .long  syscall_trace
 .Lvfork:       .long  sys_vfork
 
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
 .Lschedtail:   .long  schedule_tail
 #endif
 
diff -urN linux-2.5.6-pre2/arch/s390x/kernel/entry.S linux/arch/s390x/kernel/entry.S
--- linux-2.5.6-pre2/arch/s390x/kernel/entry.S	Tue Feb 19 21:10:58 2002
+++ linux/arch/s390x/kernel/entry.S	Mon Mar  4 17:53:31 2002
@@ -280,7 +280,7 @@
         GET_CURRENT               # load pointer to task_struct to R9
         stosm   48(%r15),0x03     # reenable interrupts
 	xc      SP_R2(8,%r15),SP_R2(%r15) # child returns 0
-#ifdef CONFIG_SMP
+#ifdef CONFIG_SMP || CONFIG_PREEMPT
 	larl    %r14,sysc_return
         jg      schedule_tail     # return to sysc_return
 #else
diff -urN linux-2.5.6-pre2/kernel/sched.c linux/kernel/sched.c
--- linux-2.5.6-pre2/kernel/sched.c	Fri Mar  1 17:21:16 2002
+++ linux/kernel/sched.c	Mon Mar  4 17:54:00 2002
@@ -397,7 +397,7 @@
 			p->sleep_avg) / (EXIT_WEIGHT + 1);
 }
 
-#if CONFIG_SMP
+#if CONFIG_SMP || CONFIG_PREEMPT
 asmlinkage void schedule_tail(void)
 {
 	spin_unlock_irq(&this_rq()->lock);