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Subject[PATCH] 2.5: preemptive kernel on UP
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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);
-
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